Hydrobiological Assessment of the
Zambezi River System: A Review
Pinay, G.
IIASA Working Paper
WP-88-089
September 1988
Pinay, G. (1988) Hydrobiological Assessment of the Zambezi River System: A Review. IIASA Working Paper. IIASA,
Laxenburg, Austria, WP-88-089 Copyright © 1988 by the author(s). http://pure.iiasa.ac.at/3116/
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WORKING PAPER
HYDROBIOLOGICAL ASSESSMENT OF THE
ZAMBEZI RWER SYSTEM: A REVIEW
September 1988
W P-88-089
l n l e r n a i ~ o n a ll n s l ~ i u l e
for A p p l ~ r d Sysiems Analysis
HYDROBIOLOGICAL ASSESSMENT OF THE
ZAMBEZI RIVER SYSTEM: A REVIEW
September 1988
W P-88-089
Working Papers are interim reports on work of the International Institute for
Applied Systems Analysis and have received only limited review. Views or
opinions expressed herein do not necessarily represent those of the Institute
or of its National Member Organizations.
INTERNATIONAL INSTITUTE F O R APPLIED SYSTEMS ANALYSIS
A-2361 Laxenburg, Austria
One of the Lmporhnt P r o j e c t s within the Environment Program i s t h a t entitled: De*on
apport *stems jbr Mancrgfnq Lurge Intemartiorrcrl Rivers. Funded by
t h e Ford Foundation, UNEP, and CNRS France, t h e P r o j e c t includes t w o case stud i e s focused on the Danube and the Zambezi r i v e r basins.
The a u t h o r of t h i s r e p o r t , Dr. G. Pinay, joined IIASA in F e b r u a r y 1987 a f t e r
completing h i s PhD at the C e n t r e dSEmlogie d e s Ressources Renouvelables in
Toulouse. Dr. Pinny was assigned t h e task of reviewing t h e published l i t e r a t u r e on
water management issues in the Zambezi r i v e r basin, and r e l a t e d ecological questions.
A t the outset, I thought that a literature review on the Zambezi r i v e r basin
would b e a r a t h e r slim r e p o r t . I am t h e r e f o r e greatly impressed with t h i s Working
P a p e r , which includes a l a r g e number of r e f e r e n c e s but more importantly, synthesizes t h e various studies and provides t h e scientific basis f o r investigating a
v e r y complex set of management issues. Dr. Pinay's review will b e a basic r e f e r e n c e f o r f u r t h e r water management studies in t h e Zambezi r i v e r basin.
R.E. Munn
Leader, Environment Program
- iii -
I would like to thank the IIASA library which i s really part of the research team
and without them this work would not have been possible. My thanks to R.E. Munn
who kindly reviewed the paper, and to Y. Taher who typed it and helped me improve
the language.
Chapter I: The Zambezi River System
The Zambezi r i v e r system les between 24-38'E
and 12-20's.
and is t h e larg-
est of t h e African r i v e r systems flowing into t h e Indian Ocean [(Balek 1977; Davies
1986). Figure
11.
I t consists of t h r e e sections (Balon and Coche 1974): ljrlpper from
its sources to t h e Victoria Palls, Middle from t h e Victoria Palls to t h e Cahora
Bassa rapids, and Lower from Cahora Bassa to the Indian Ocean. These t h r e e
stretches s e e m to have been independent until t h e Pliocene era (Balon 1978) following which. due to tectonic movements, they merged to form t h e actual r i v e r sys-
t e m (Table 1).
There is much evidence to support such a picture of evolution particularly in
relation with pre-impoundment fish distributions in t h e r i v e r (Jackson 1986). The
length of t h e r i v e r itself is controversial. For instance, Welcomme (1977) claims i t
is 2574 km, Balon and Coche (1974) 2494 km, Balek (1977) 2600 km, and Beadle
(1932) 3000 km. I t flows eastwards from its sources in the Central African Plateau
at 1400 m altitude to t h e Indian Ocean (Figure 2). The surface of the Zambezi
drainage basin varies greatly from 1193500 km2 (Balon and Coche, 1974) to
1570000 kmz (Balek 1977). In addition, one can divide t h e entire Zambezi drainage
basin into subcatchments a r r e s p o n d i n g to the drainage basins of the main tributaries of t h e Zambezi r i v e r m b l e 2).
The Zambeei drainage basin is situated south of t h e equator between 12' and
20°S.
The cool dry season is between May end September.
The headwaters
situated north of the basin belong to the tropical summer rainfall zone (climate
m e I1 in W a l t e r et
a.1973).
As one moves south, t h e tropical summer becomes
progressively more arid due to a prolongation of the dry season (Pigure 3). Thus,
t h e upper and t h e middle Zambezi are defined as warm temperate regions with dry
winter, t h e warmest month being up to 22' C (Schulze and McGee, 1978) a s quoted by
Davies (1979). while t h e Okavango basin and t h e southern part of t h e middle
F
m 1. The Zambezi drainage basin.
%ble 1. Main f e a t u r e s of t h e t h r e e s t r e t c h e s of t h e Zambezi r i v e r
ZAMBEZI
STRETCH
Source
LENGTH
km
DRAINAGE SURFACE
km2
MEAN ANNUAL FLOW
m3 s - I
UPPER
Victoria Falls
M l DDLE
Cahora Bassa
rapids
853
LOWER
Indian Ocean
Table 2. Main subcatchments of t h e Zambezi r i v e r system.
1
I
basin km
)
Chobe
95800
I
Annual
3
flow km
A k W
1
1
Sanyati
43500
I
4.2
Qvrum
Fdls
/
1
Kafue
Huangwa
154200
147500
I
3.4
1
Hunyani
23900
I
10.3
13.0
1
I
Shire
115000
I
1.4
N.D.
V i *
1100
124)
Q)O
a00
an
an
34)
0
UPPER
MIDDLE
0
LWER
zwl
250d km length
Figare 2. Longitudinal profile of t h e Zarnbezi River.
Zambezi belong to t h e a r i d s t e p p e zone with a mean annual temperature of more
than 18°C. Finally, t h e lower Zambezi moves from t h e a r i d climatic zone in t h e w e s t
to a n equatorial climate with d r y winters in t h e east.
Figure 9. Map showing precipitation repartition on t h e Zambezi drainage basin
adapted from Vieira (1961)and Bolton (1983).
2. Pain Water Retention Systems
S e v e r a l kinds of water s t o r a g e systems such as natural lakes, man-made lakes,
and swamps exist along t h e Zambezi r i v e r and i t s tributaries.
2.1. Natural lakes
The only l a r g e n a t u r a l l a k e on t h e Zambezi watershed is Lake Malawi. I t i s t h e
t h i r d l a r g e s t lake in Africa with a maximum length of 580 km and a width of between
16 and 80 km, and a s u r f a c e of about 30000 km2. I t s volume capacity i s almost
8000 km3. The hydrological balance of t h e lake i s dominated by permanent rainfall
which r e a c h e s 2270 mm p e r annum in t h e l a k e area. The lake i s subject to a b r u p t
annual and seasonal changes in i t s water level as a r e s u l t of variations in rainfall.
The S h i r e r i v e r in t h e southern p a r t of t h e lake constitutes i t s outlet; this r i v e r
f l o w s through a small lake, t h e Malombe, before i t joins t h e Zambezi in its lower
reach.
2.2. Ban-Ymdc hkam
Two man-made lakes, t h e Kariba and Cahora Bassa, are on t h e Zambezi r i v e r
regulating almost 570 km of t h e length of t h e r i v e r . These impoundments r e p r e s e n t
65 p e r c e n t of t h e middle Zambezi. Some o t h e r man-made l a k e s lie on t h e Zambezi
t r i b u t a r i e s , mainly in Zambia on t h e Kafue r i v e r a n d in Zimbabwe. The c h a r a c -
teristic f e a t u r e s of t h e l a r g e r l a k e s are given in C h a p t e r 111, while t h e main
f e a t u r e s of t h e most important o n e s are given in Table 3 (Marshall a n d Falconer
1973a-b; Mitchell a n d Marshall 1974; Jackson and Davies 1976) and t h e i r locations
in Figures 1and 4.
Figure 4. Locations of man-made lakes mound H a r a r e , Zimbabwe.
Certain c h a r a c t e r i s t i c s of man-made l a k e s are t r p i c a l f o r most of them, and,
t h e r e f o r e , are distinguishable from n a t u r a l l a k e systems. The development of t h e
s h o r e l i n e is slightly h i g h e r t h a n t h a t f o r n a t u r a l lakes; if t h e r i v e r i s confined
Table 3. Hydrological f e a t u r e s of t h e main r e s e r v o i r s of t h e Zambezi drainage
basin.
I
I
Lake s u r f a c e
(kmZ)
Drainage basin
(km2)
Kariba
Cahora Bassa
Ithezithezi
Kafue Gorge
McIlwaine
Mazoe
Mwenie
between high banks, t h e new l a k e will b e long and narrow; if t h e r e are t r i b u t a r i e s ,
water will b e held back giving t h e new lake a dendrite form. In comparison, t h e
e x t e n t of shoreline modification in a r e s e r v o i r is g r e a t e r than in a natural lake
because t h e annual drawdown exposes a l a r g e r area t o t h e effects of s h o r e
processes. Whereas normally natural l a k e s are deepest n e a r t h e c e n t r e , man-made
lakes are almost always deepest just upstream from t h e dam.
They are then
r e f e r r e d to as "half-lakes" (Ellis 1941). The incoming water does not mix immediately with t h e water in t h e r e s e r v o i r due to differences in temperature and cont e n t of dissolved or suspended solids. I t leads to density c u r r e n t s and hypolimnetic
withdrawals (Baxter 1977). The retention time in man-made lakes is v e r y s h o r t
compared to t h a t of n a t u r a l lakes. For instance, t h e retention time of water in t h e
Kariba w a s measured at f o u r y e a r s , while i t i s about one y e a r for t h e Cahora
Bassa.
According to Wetzel (1975) a swamp i s a more or less permanently waterlogged system with persistent standing water in t h e vegetation. In Africa h e r b a ceous swamps may b e defined as f l a t areas t h a t are flooded to a shallow depth
densely covered with herbaceous vegetation whose shoots r i s e above t h e water
level to more t h a n one metre. The swamps are bottom-rooted or floating (HowardWilliams and Gaudet 1979). T h e r e are s e v e r a l reasons why swamps should b e taken
into consideration f o r t h e i r hydrological and biological processes. Among them
water retention, water loss by evapotranspiration, t h e sedimentation process,
fishery r e s o u r c e s , and grazing p a s t u r e s may b e mentioned. The p r e s e n t c h a p t e r
p r e s e n t s some f e a t u r e s of t h e main swamps in t h e Zambezi drainage basin, and t h e
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2.3.1. Barotse swamps
The B a r o t s e floodplain l i e s on t h e u p p e r c o u r s e of t h e Zambezi r i v e r in t h e
Central African plateau in western Zambia. It i s a 240 km long a n d 40 km wide
floodplain through which t h e Zambezi r i v e r meanders. The annual rainfall i s a b o u t
1 0 5 0 mm of which 9 0 p e r c e n t o c c u r s between November a n d March. Flooding of
t h e u p p e r Zambezi normally commences in November, b u t may v a r y depending on
t h e intensity of t h e e a r l y r a i n s in t h e headwater catchments. Usually t h e flood
p e a k s are in April in t h e B a r o t s e floodplain, a n d r e c e d e slowly o v e r a p e r i o d of six
months between May a n d October (FAO-UN 1969; Burgis a n d Symoens 1987).
2.3.2. Okavango delta
The Okavango d e l t a system i s r a t h e r complex b e c a u s e of i t s hydrological
f e a t u r e s leading to g e n e r a l flow p a t t e r n s as well as sedimentation p r o c e s s e s t h a t
c h a n g e t h e delta's topography. Surprisingly t h e Okavango d e l t a h a s a r e g u l a r
conical s h a p e with a s l o p e of a b o u t 1in 3600. Thus, relatively small i r r e g u l a r i t i e s
provide t h e p a t t e r n of channels. ridges, swamps a n d pools. Estimates of t h e area
of t h e Okavango swamp v a r y between 1 0 000 a n d 1 8 0 0 0 km2, b u t t h e estimation of
Wilson a n d Dincer (1976) from s a t e l l i t e imagery i s 1 0 0 0 0 km2. The water balance
of d e l t a i c swamps d i f f e r s from usual r i v e r basins or l a k e s , mainly by varying
swamp areas a n d flow distribution. Thus, hydrological f e a t u r e s v a r y l a r g e l y as
functions of water input (precipitation o n t h e d r a i n a g e basin) and output (evapot r a n s p i r a t i o n , runoff).
The Okavango catchment consists mainly of two r i v e r systems, t h e Cubango
with a catchment area of 1 1 5 0 0 0 km2 with a n a v e r a g e annual rainfall of 983 mm,
a n d t h e Cuito with a d r a i n a g e basin of 65000 km2 and a n a v e r a g e annual rainfall of
876 mm [(Wilson a n d Dincer 1976) Figure 63. The t w o r i v e r s then merge to form t h e
Okavango r i v e r b e f o r e e n t e r i n g t h e Okavango delta.
Usually, t h i s input of water is completely r e t a i n e d in t h e Okavango d e l t a
2
w h e r e evapotranspiration i s t h e main p r o c e s s (about 2 8 3 0 mm/year/m ). E x c e s s
water flows t h r o u g h t h e Boteti Channel to Makgadikgadi P a n s system. During high
rainfall y e a r s t h e Okavango swamps system c a n d r a i n t h e e x c e s s water through
Selinda spillway to t h e Chobe r i v e r , a n d t h e n o n to t h e Zambezi r i v e r (Figure 6).
The Selinda spillway i s t h e only link between t h e Okavango d r a i n a g e basin and t h e
Zambezi, which means t h a t t h e Okavango d r a i n a g e basin i s r e l a t e d t o t h e Zambezi
r i v e r only during t h e heavy r a i n f a l l season. Wilson a n d Dincer (1976) have given a
Figure 6. Drainage basin of t h e Okavango d e l t a and i t s connection with t h e Zambezi r i v e r system.
t e n t a t i v e water balance f o r t h e Okavango d e l t a (Table 5).
TPrble 5. Water balance f o r t h e Okavango d e l t a (Wilson a n d Dincer 1976).
Processes
Inflow of Okavango r i v e r
Precipitation
Total
I Annual input I
Annual output
11 X 109m3
5x 10)~~
16 x 10 m
1 6 x 109m3
Evapotranspiration
Outflow (Boteti r i v e r )
Groundwater outflow
To complicate t h e hydrology, t h e function of t h e Chobe r i v e r is to r e v e r s e t h e
flow when t h e u p p e r Zambezi i s flooded (Davies 1986). Although t h e Okavango
d r a i n a g e basin (almost 200000 km2) is a p a r t of t h e Zambezi d r a i n a g e basin, i t h a s
n o t t h e e x p e c t e d e f f e c t s o n t h e Zambezi r i v e r itself (runoff) d u e to t h e b u f f e r
e f f e c t of t h e Okavango d e l t a a n d t h e unstable connection between them.
2.3.3. Kafue armmps
The Kafue r i v e r i s t h e l a r g e s t t r i b u t a r y of t h e Zambezi r i v e r . Almost 1 5 0 0 km
in length i t d r a i n s a n area of about 155000 km2. Three main swamps make up t h e
drainage basin (Figure 7).
Lukanga swamp
This occupies a shallow depression extending to about 2600 km2, but 2 100 km2
only i s permanent swamp. The a v e r a g e water capacity i s estimated at 7.38.10 m3
(Burgis and Symoens 1987). I t r e c e i v e s water from s e v e r a l catchment streams as
well as from t h e Lukanga r i v e r , a t r i b u t a r y of t h e Kafue, which drains a n area of
1 4 2 4 5 km2. During high water level periods, i t also r e c e i v e s water from t h e Kafue
r i v e r itself.
Busanga swamp
This comprises about 1 0 0 0 km2 along t h e Lufupa r i v e r , a t r i b u t a r y of t h e
Kafue r i v e r . Although i t i s not well known ecologically n o r i s i t exploited because
it i s in t h e tsetse fly zone, t h i s swamp i s similar to t h e Lukanga swamp f r o m t h e
physiographical point of view (Burgis and Symoens 1987).
K-e
flats
A t Itezhitezhi t h e Kafue r i v e r b r e a k s through a r a n g e of l o w hills b e f o r e i t
flows eastward across t h e Kafue flats. Here i t meanders f o r 410 km traversing a
distance of 250 km across t h e floodplain at a n a v e r a g e gradient of only 2.7cm/km
(White 1973; Dudley 1974). During i t s c o u r s e through t h e flats, t h e r i v e r falls only
15 m in 410 km (Rees 197th). After flowing through t h e floodplain, t h e r i v e r
plunges down 670 m as i t flows through t h e 30 km Kafue Gorge to t h e Zambezi
river.
I t i s typical f o r t h e waters of t h e Kafue r i v e r to start to r i s e in l a t e
November or e a r l y December shortly a f t e r t h e start of t h e rainy season. The
highest water levels are in April/May, about o n e month a f t e r t h e termination of t h e
local rainfalls (Dudley 1979), when about 5 6 5 0 km2 may b e inundated; t h e water
recedes slowly until November. The natural hydrological cycle of t h e Kafue f l a t s
v a r i e s tremendously. Three-quarters of t h e floodplain c a n change from a n aquatic
to a t e r r e s t r i a l environment within one season. A t t h e e a s t e r n end of t h e Kafue
flats, a shallow area of about 1 2 1 5 km2 i s flooded more or less permanently (Burgis
and Symoens 1987). The major input of water to t h e system i s f r o m t h e headwater
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Figure 7. Location of swamps on the Kafue river system (Burgis and Symoens,
1987).
areas of t h e Kafue r i v e r above Itezhitezhi (controlled by a dam since 1978), d i r e c t
rainfall, and seasonal runoff from t h e surrounding higher grounds. Water is lost
by discharge through t h e Kafue Gorge (regulated by a dam since 1972) and by evapotranspiration.
A tentative water balance in t h e Kafue r i v e r system is given by Burgis and
Symoens (1977) (Figure 8). Thus, i t has been estimated t h a t from t h e e n t i r e runoff
of t h e Kafue drainage basin, which is about 1 2 . 1 6 . 1 0 ~ m ~7. 3 p e r c e n t ( o r
8.88.109m3) r e a c h e s t h e Kafue Rail Bridge (downstream Kafue flats). The remaining 27 p e r c e n t ( o r 3.28.109m3) is believed to be lost through evapotranspiration
jointly from t h e Kafue floodplains and t h e Lukanga and Busanga swamps.
Figure 8. Water balance d t h e Kafue r i v e r system (Burgis and Symoens, 1987).
A review of t h e examples cited above, mainly of t h e Okavango delta and t h e
Kafue flats f o r which more d a t a are available, shows t h e importance of swamps in
t h e water balance of an e n t i r e drainage basin, especially in those tropical regions
where t h e evapotranspiration process is intensified.
3. Flow Regime of the Zambezi
The general f e a t u r e of hydrological cycles in t h e t r o p i c s i s v e r y similar to
those in more moderate regions, although several factors affecting water movement from t h e atmosphere through and o v e r t h e e a r t h ' s s u r f a c e into t h e r i v e r s are
of p a r t i c u l a r importance, and t h e resulting effects deviate f r o m t h a t experienced
outside t h e tropics. For instance, t h e specific function of interception, different
time and space distribution of rainfall, t h e pronounced influence of swamps, and
soil problems are e x a c e r b a t e d in these regions (Balek 1977). global water balance
f o r t h e e n t i r e Zambezi r i v e r i s estimated and evaluated for t h e different subcatchment basins corresponding to t h e t h r e e s t r e t c h e s identified above (Table 6).
%ble 6. Water balance of t h e Zambezi drainage basin (Balon and Coche 1974;
UNEP 1986a; Davies 1986).
Zambezi
stretches
Upper
Middle
Lower
Drainage basin
(km2>
320 000
1118000
1400 000
Precipitation
(km3>
360
830
1317
Evapotrans iration
(km )
245
688
1000
?
Runoff
(km3>
49.2
74.8
106.4
A s shown above, water retention systems like swamps and man-made lakes play
a c r u c i a l role in t h e global water balance of t h e Zambezi r i v e r system, also in t h e
dynamics of t h e flow itself.
For t h e s a k e of presentation and because i t
corresponds t o reality from t h e hydrological point of view, t h e Zambezi r i v e r will
be examined in t h r e e sections which more or less coincide with t h e t h r e e natural
stretches.
3.1. The Upper Stretch
The u p p e r s t r e t c h corresponds to t h e section between t h e s o u r c e of t h e Zambezi in a marshy bog n e a r t h e Kalene hills about 1 5 0 0 m above sea-level and t h e
Victoria Falls n e a r t h e town of Livingstone. The 1 6 5 0 wide Victoria Falls with a
mean annual discharge of 1 2 3 7 m3/s form a 98 m high b a r r i e r between t h e u p p e r
and t h e middle Zambezi. The u p p e r s t r e t c h i s t h e most natural to t h e Zambezi
r i v e r owing to t h e absence of impoundments. Rainfall in t h e drainage basin r a n g e
from 1 4 0 0 mm/year in t h e headwater zone to 700 mm southward. Temperatures
r e a c h 20-22°C with a r a n g e of 6-8" C. The rainy season i s between November and
April, and t h e d r y season from May to July. The double flood peak r e c o r d e d during
t h e rainy season upstream from t h e Barotse is balanced by t h e d r y season. The
resulting single flood peak due t o t h e buffer effect of t h e Barotse swamps i s then
f u r t h e r equalized by t h e Chobe swamps (Balon and Coche 1974).
Figure 9. Average flow regime (1925-1966) a t Livingstone, and flow regime during
a d r y (1949-1950) and a w e t (1957-1958) period (Balon and Coche, 1974).
Next, a c h a r a c t e r i s t i c flood peak i s observed at Livingstone. Figure 9 gives
t h e a v e r a g e monthly runoff at t h e Livingstone pump station calculated from t h e
d a t a obtained between 1925 and 1966. The flow starts to i n c r e a s e in December t o
r e a c h a peak in April (3500 m3/s), then i t d e c r e a s e s to about 350 m3/s in October.
However, because of t h e e r r a t i c rainfall p a t t e r n of t h e region, t h e Zambezi flow
v a r i e s greatly from y e a r to y e a r . Table 7 gives t h e frequency p e r month of t h e
minimum or maximum discharge.
Thus, Balon (Balon and Coche 1974) h a s estimated t h a t i t normally t a k e s
approximately t h r e e to f o u r weeks f o r t h e base flood peak to move from Balovale,
upstream to Barotse swamps, to Kariba. The annual runoff from t h e u p p e r Zambezi
Tcrble 7. Frequency of minimum and maximum d i s c h a r g e between 1 9 2 5 and 1966
(Balon a n d Coche 1974).
Minimum d i s c h a r g e
Maximum d i s c h a r g e
Months
Frequency
Months
Frequency
September
October
November
December
9
26
20
1
February
March
April
Hay
June
2
11
23
6
1
catchment during t h e p e r i o d 1924-1979 (Figure 1 0 ) shows a r i s e during t h e l a t e
1940s and e a r l y 1950s. According to DuToit (1982), only 30 p e r c e n t of t h e
i n c r e a s e could b e explained by a n i n c r e a s e of rainfall o v e r t h e catchment a r e a .
Various explanations f o r t h e i n c r e a s e in r i v e r flow h a v e been suggested, such as
changes in rainfall p a t t e r n s (spatial a n d seasonal distribution of mean water
d e p t h s as well as intensities) a n d c h a n g e s in t h e watershed r e s p o n s e (Puzo 1978).
To s u p p o r t t h i s idea, DuToit (1982) o b s e r v e d t h a t t h e v a r i a t i o n s of t h e mean
annual d i s c h a r g e became m o r e pronounced in t h e y e a r s 1950-1980, suggesting
t h a t t h e s u r f a c e runoff i n c r e a s e d at t h e e x p e n s e of t h e underground water t a b l e
r e c h a r g e a n d increasing t h e d i f f e r e n c e s in runoff between w e t and d r y y e a r s .
Undoubtedly, f u t u r e man-made d r a i n a g e of t h e Barotse and Chobe swamps would
promote t h i s e f f e c t .
O t h e r hypotheses c o n c e r n changes in t h e f e a t u r e s of t h e catchment r a t h e r
t h a n c h a n g e s in t h e rainfall (Puzo 1978). Variations in t h e annual flow from high to
low flow y e a r s h a v e become m o r e pronounced, which might suggest t h a t when r a i n
d o e s o c c u r , i t flows off t h e land rapidly d u e to superficial c h a n g e s r a t h e r t h a n
percolation, maintaining a supporting flow through a following d r i e r y e a r (DuToit
1982). Possible d r a i n a g e of Barotse a n d Chode swamps should heighten t h i s e f f e c t .
In a n y c a s e , t h e problem remains unsolved. Since 1980-1981, t h e u p p e r Zambezi
catchment h a s e x p e r i e n c e d a n unprecedented d r o u g h t causing o t h e r problems f o r
t h e management of t h e Kariba R e s e r v o i r .
Figure 10. Annual r u n o f f s of t h e u p p e r Zambezi oatchment during the period
1924-1979 (five y e a r s running means, DuToit, 1983).
3.2. The Middle Stretch
The n a t u r a l r i v e r situation of t h e middle as well as t h e lower s t r e t c h e s h a s
changed drastically by c r e a t i n g l a k e s Kariba a n d Cahora Bassa, both within t h e
flooded area a n d downstream from t h e dams. Before t h e impoundments, t h i s r i v e r
was defined as a "sand-bank" r i v e r , like t h e Huangwa a n d t h e Limpopo r i v e r s , by
Jackson (1961). Due to t h e c r e a t i o n of t h e two man-made lakes, t h i s p a r t of t h e
r i v e r should now b e considered as a " r e s e r v o i r r i v e r " , such as t h e Kafue o r t h e
S h i r e r i v e r s . In i t s n a t u r a l s t a g e , t h e middle Zambezi h a s had two high flood
p e r i o d s e v e r y y e a r (Figure 11): t h e "Gumbura" as called by t h e local people comes
usually in F e b r u a r y . with l e s s t u r b u l e n t flood waters, c a r r y i n g local runoff. The
second high flood, known as the"Murorwe" used to come in April, but s i n c e t h e
K a r i b a w a s constructed, i t n o l o n g e r inundates t h e floodplain below t h e wall.
One c a n notice t h a t t h e Kafue catchment runoff joining t h e Zambezi down-
stream and t h e K a r i b a i s flattened d u e to a n accentuation of t h e n a t u r a l " r e s e r v o i r
r i v e r " of t h e Kafue r i v e r by i t s impoundments (Itezhitezhi, Kafue Gorge dams).
Figure 12 r e p r e s e n t s t h e situation in r e c e n t y e a r s f o r t h e monthly inflows a n d spill a g e s f o r Lake Kariba. This g r a p h r e v e a l s t h e f a c t t h a t f o r t h e p e r i o d considered,
t h e K a r i b a scheme d o e s not s e e m to h a v e r e g u l a t e d t h e Zambezi flow on a monthly
s c a l e , although t h e inflow i s somewhat g r e a t e r t h a n t h e outflow during t h e f i r s t
p a r t of e v e r y r a i n y season, a n d t h e outflow is somewhat g r e a t e r t h a n t h e inflow
during t h e d r y season. On t h e o t h e r hand, t h e Kariba scheme h a s r e g u l a t e d well
months
Figure 11. Mean monthly flows upstream from Kariba f o r t h e period 1924-1979
(DuToit, 1982).
t h e s h a r p flood peaks of 1963,1969 and 1970. For instance, t h e flood peak at Vict o r i a Falls in 1969 was estimated at 8100 m3/s, while t h e spilling through t w o
floodgates amounted no more than 3000 m3/s.
3.3. The h e r Stretch
Before t h e Kariba impoundment w a s built, t h e Zambezi showed a r e g u l a r
annual cycle, usually reaching i t s peak in February or March at 5000-20000
m3/sec and falling to 200-800 m3/sec in October-November.
t h e flow into t h e Cahora Bassa r e s e r v o i r is regulated.
Now 90 p e r c e n t of
The Kariba dam has
resulted in an i n c r e a s e in d r y season flows and a delay in t h e timing of floods
Figure 12. Monthly inflow and outflow f o r Kariba from October 1976 to September
1979 (DuToit, 1982).
during t h e w e t season (Bernacsek and Lopes 1984). The flood magnitude h a s been
decreased by a n a v e r a g e of 24 p e r c e n t during eight y e a r s in t h e 19'70-80 period.
The e r r a t i c water management of Cahora Bassa h a s completely a l t e r e d t h e normal
water flow downstream to t h e floodplain and t h e delta. A s shown in Figure 13,maximum discharges almost always t a k e place out of season. Most y e a r s tend to have
t w o flood releases, and in some cases even t h r e e as in 1981 (Figure 13).
Since t h e closure of t h e Cahora Bassa dam in December 1974,a period of t w o
y e a r s (1979 and 1980) may b e considered as "normal" from t h e hydrological and
operational perspectives, although even during t h i s period t h e Zambezi hydrologi-
cal cycle w a s tremendously disturbed (Figure 14) with a retention of flood peak in
t h e r e s e r v o i r between February and May, creating t w o artificial flood peaks out of
season.
8. m
16 m
>
ram
IL
.m
b
m
i: ,lorn
.,
1
IP
P w e 13. Water flow through Cahora Bassa reservoir. (A) total monthly inflow
and outflow volume; (B) difference in monthly inflows (Bernacsek and Lopes, 1984).
I
O
I
I
N
O
I
J
I
F
I
M
I
A
I
M
I
J
J
I
I
A
S
I
~
nm
Figure 14. Water flow Ulrough Cahora Bassa reservoir in 1980. Shaded area
represents buffering e f f e c t of the lake during floods.
3.4. Conclusions
As shown above, now t h e Zambezi i s f a r from from t h e description of Jackson's
in 1 9 6 1 as having sand-bank c h a r a c t e r i s t i c s : ' k a v e v e r y d e e p beds with welldefined s t e p banks c u t in t h e alluvial e a r t h , through which t h e r i v e r s meander at
l o w level, often little more t h a n a connection between a s e r i e s of pools fringed with
r o c k s and sand b a n k s with l i t t l e aquatic vegetation, and still less marginal vegetation on t h e banks, while t h e flood i s violent but of comparatively s h o r t duration,
with t h e floodplains inundated f o r a relatively s h o r t s p a c e of time." (Jackson 1961)
The impoundment of t h e middle a n d lower Zambezi a n d i t s main t r i b u t a r i e s (Kafue
a n d S h i r e ) h a s transformed t h i s r i v e r to a " r e s e r v o i r system" dependent on
anthropogenic management.
The international s t a t u s of
t h e Zambezi r i v e r
escalates t h e problems r e l a t e d to i t s management. As underlined by Bernacsek and
Lopes (1984) f o r instance, d e s p i t e t h e f a c t t h a t about 9 0 p e r c e n t of t h e inflow into
Cahora Bassa r e s e r v o i r is r e g u l a t e d , major problems in flood prediction and
management remain.
The following c h a p t e r i s devoted to t h e s e international
c h a r a c t e r i s t i c s of t h e Zambezi r i v e r basin.
Chapter II: The Zambezi Drainage Basin
1. Political Boundaries
The Zambezi drainage basin i s s h a r e d by eight countries of t h e southern Afric a n region. They are Angola, Botswana, Malawi, Mozambique, Namibia, Tanzania,
Zambia and Zimbabwe (Figure 15). According to t h e i r geographical locations, they
s h a r e t h e catchment with a l a r g e r a n g e of s u r f a c e contributions. Table 8 surnrnarizes t h e contributions of t h e eight countries to t h e Zambezi drainage basin.
Table 8. S h a r e of t h e Zambezi drainage basin by t h e eight countries.
Total s u r f a c e
km2
Countries
I
1I
I
S u r f a c e in t h e
watershed
km2
% watershed
Angola
Botswana
Malawi
Mozambique
Namibia
Tanzania
Zambia
Zimbabwe
The concept of t h e drainage basin means t h a t all land drained to a r i v e r o r i t s
t r i b u t a r i e s belongs t o t h e catchment of t h i s r i v e r . One must also consider t h a t
some countries do not have d i r e c t contact with t h e Zambezi r i v e r itself, but t h a t
p a r t of t h e t e r r i t o r y i s drained by a t r i b u t a r y of t h e Zambezi. This i s t h e case in
Malawi and Tanzania t h a t are drained towards Lake Malawi and t h e S h i r e (Figure
15). These boundary differences among countries entail different i n t e r e s t s in t h e
Zambezi r i v e r . Complicating t h e political situation in each country as well as f o r
international negotiations, many ministries s h a r e water administration and management in e a c h nation. Table 9 gives t h e number of ministries concerned with water
management in e a c h country.
f'l
ZAMBEZI D R A I N A G E BASIN POLITICAL BOUNDARIES
Figure 15. Political boundaries of t h e Zambezi drainage basin.
TabLe 9. Number of ministries sharing water administration in t h e Zambezi countries.
Countries
Angola
Botswana
Malawi
Mozambique
Namibia
Tanzania
Zambia
Zimbabwe
Number of ministries
ND
6
5
6
ND
8
7
8
2. Economic Situation
2.1. Population
2
Within i t s area of about 1400000 km , t h e Zambezi d r a i n a g e basin h a s more
t h a n 20 million inhabitants. Table 10 gives t h e population distribution within t h e
catchment f o r each country.
Among them Malawi, Mozambique, Zambia and Zimbabwe account f o r 94.70 p e r
c e n t of t h e population in t h e basin, while t h e o t h e r s s h a r e 5.3 p e r c e n t although
t h e drainage area they c o v e r i s a q u a r t e r of t h e t o t a l Zambezi catchment. The
g e n e r a l p a t t e r n i s towards a n i n c r e a s e of population in e a c h country since 1965,
and a d e c r e a s e in p e r c e n t a g e of population economically a c t i v e in a g r i c u l t u r e
(Figure 16). Nonetheless, more t h a n 70 p e r c e n t of t h e population is involved in
a g r i c u l t u r e in a l l t h e r i p a r i a n countries, e x c e p t Botswana (66 p e r c e n t ) and Namibia (38.3p e r cent).
TabLe 10. Population distribution of t h e Zambezi d r a i n a g e basin.
Angola
Botswana
I1
poiEion
,
/
1
Z of t o t a l
X of basin
population , population
Basin
population
,
1
8900000
303740
3.41
1149000
8100)
0.70
Malawi
7178000 ( 7178000
100
Mozambique
14342000
2566708 (
17.89 (
I
Namibia
1596000
40010 1
2.50 1I
Tanzania
23334000
815420
3.49
Zambia
6898000
4482396 (
70.2
Zimbabwe
9099000
65603791
72.1
p
Total
, 72496000 , 21954753 1
30.3
I
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
!
1.38
0.04
32.70
11.69
0.18
3.71
20.42
29.88
p
100
v a s t region generally with sandy soil. Predominant c r o p s are sorghum and bulrush
millet. (c) E a s t e r n Cuando-Cubango i s a region of p o o r soil and s p a r s e population.
Subsistence farming and food collecting are dominant with pastoralism in t h e d r y
s o u t h e a s t e r n area.
Figure 16. Evolution of population in t h e Zambezi riparian countries (.-.) and
percentage of population economically active in agriculture (.---.) (FAO, 1986).
2.2. Land-Use
For each country sharing t h e Zambezi r i v e r basin, t h e statistics shown below
were taken from t h e FA0 Production Yearbook (1977-1987) concerning t h e whole
country, and not just t h e p a r t belonging to t h e Zambezi catchment. Nevertheless,
t h e s e d a t a provide a quite a p p r o p r i a t e idea of t h e situation on t h e drainage basin,
at l e a s t f o r Malawi, Mozambique, Zambia and Zimbabwe. Concerning t h e o t h e r
countries* it shows at l e a s t b e main f e a t u r e s and tendency.
2.2.1. Angola
Since 1965, statistics show a n increase in a r a b l e land mainly between 1970 and
1975 which is certainly due t o increased deforestation during t h e s a m e period.
However, permanent c r o p area has not increased. The p a r t of Angola belonging to
t h e Zambezi drainage basin c o v e r s t h r e e regions: (a) East Central Moxico, with
r i v e r basins subject to flooding, and villages located on low elevations. Hoe cultivation of subsistence c r o p s i s done during t h e rainy season. The region close to
t h e Zambezi r i v e r i s a more favoured area f o r c r o p farming with relatively import a n t stock raising, e x c e p t in t h e tsetse-infested zones. (b) Southern Moxico is a
Table U . Land-use in Angola. Statistics in km2.
Land-Use
!
1965
!
1970
/
1975
!
1980
!
1985
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
2.2.2. Botswana
A s shown above, Botswana's area belonging to t h e Zambezi drainage basin is
mainly r e p r e s e n t e d by t h e Okavango region.
In Botswana, t h e r e is a major study called "Southern Okavango Integrated
Water Development Study" under preparation. I t seems t h a t s o m e 150 krn2 would be
suitable for large-scale irrigation f o r agricultural development (UNEP 1986b).
The Okavango delta i s a l s o subject to large-scale c a t t l e raising, mainly in t h e
southern p a r t of t h e delta. Development of t h i s region i s due partly to t h e eradication of t h e tsetse fly which h a s allowed settlement by people.
Table 12. Land-use in Botswana. S t a t i s t i c s in km2.
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
2.2.3. Malawi
Almost 1 0 0 p e r c e n t of Malawi t e r r i t o r y belongs to t h e Zambezi d r a i n a g e
basin.
Table 23. Land-use in Malawi. S t a t i s t i c s in km2
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
Tea a n d t o b a c c o are t w o main c r o p s t h a t c o v e r 210 and 900 km2, r e s p e c t i v e l y .
Coffee now s u r v i v e s l a r g e l y as a small-holder c a s h c r o p in t h e n o r t h . Commercial
s u g a r production i s impossible without i r r i g a t i o n . More a n d more land i s becoming
permanently cultivated a n d u s e of a r t i f i c i a l f e r t i l i z e r i s increasing, although no
f e r t i l i z e r is used to grow c a s s a v a , millet, maize, sorghum, cotton a n d r i c e , e x c e p t
when c o t t o n a n d r i c e are grown in organized land schemes w h e r e f e r t i l i z e r u s e i s
encouraged.
2.2.4. Mozambique
With a b o u t 20 p e r c e n t of i t s t e r r i t o r y , t h e area belonging t o t h e Zambezi
d r a i n a g e basin h a s g r e a t a g r i c u l t u r a l potential. Thus, almost 2 5 0 0 km2 were identified as potential a r a b l e land mainly on t h e Zambezi flood plains.
Table 14. Land-use in Mozambique. S t a t i s t i c s in km2.
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
Tete highlands r e g i o n i s predominantly humid and t e m p e r a t e with mainly ferr a l l i t i c soils on which maize and p o t a t o e s are cultivated. Close t o t h e Zambezi
r i v e r , soils are predominantly brown soils with sorghum, millet, maize a n d cotton
production. The main t y p e s of farming in t h e r e g i o n s belonging t o t h e Zambezi
watershed are semi-subsistence farming a n d subsistence farming with livestock.
2.2.5. Namibia
The area of Namibia belonging t o t h e Zambezi d r a i n a g e basin i s called t h e
Caprivi s t r i p , a region v e r y s p a r s e l y populated. S t a t i s t i c s f o r t h e whole c o u n t r y
given in Table 15 cannot b e t a k e n as r e p r e s e n t a t i v e of t h e Caprivi s t r i p land use.
Table 25. Land-use in Namibia. S t a t i s t i c s in km2.
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
1
ND
ND
1
35000
60
1
32000
70
1 29000
80
26000
80
2.2.6. Tanzania
Almost t h r e e p e r c e n t of Tanzania t e r r i t o r y which i s d r a i n e d to L a k e Malawi i s
still at i t s e a r l y s t a g e of a g r i c u l t u r a l development. A s shown in Table 1 6 , i t i s not
t h e case f o r t h e main p a r t of t h e t e r r i t o r y . In t h i s small a r e a , i r r i g a t i o n p r a c t i c e s
are not well developed. E x c e p t f o r t o b a c c o , n o s u b s t a n t i a l c a s h c r o p i s grown, b u t
normal s u b s i s t e n c e c r o p s are p r o d u c e d .
Table 16. Land-use in Tanzania. S t a t i s t i c s in km2.
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
2.2.7. Zambia
In Zambia, t h e t o t a l area of a r a b l e land i s estimated at a b o u t 1 0 million hect a r e s . A t p r e s e n t , t h e development of t h i s r e s o u r c e b a s e h a s high p r i o r i t y in t h e
national development planning.
Although t h e amounts v a r y c o n s i d e r a b l y f r o m y e a r t o y e a r , maize, groundnuts
a n d c o t t o n are most i m p o r t a n t sectors of involvement. T h e t w o main t y p e s of
t i m b e r are p i n e a n d e u c a l y p t u s f o r u s e in t h e mining a n d c o n s t r u c t i o n i n d u s t r i e s .
Commercial cultivation i s mainly a l o n g t h e Kafue r i v e r system a n d t h e s o u t h e r n
agricultural belt.
Table 17. Land-use in Zambia. S t a t i s t i c s in km2.
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
2.2.8. Zimbabwe
Due to i t s dominance in t h e economy, t h e government h a s d e c l a r e d t h e
development of a g r i c u l t u r e as t h e highest p r i o r i t y in t h e c u r r e n t five y e a r s
national development plan (UNEP 1986b). One principal means of increasing a g r i c u l t u r a l production is to i n c r e a s e t h e t o t a l area of cultivated land mainly in t h e
Zambezi d r a i n a g e basin.
Table 28. Land-use in Zimbabwe. S t a t i s t i c s in km2.
Land-Use
Arable
Permanent
crop
Permanent
pasture
Forest
Irrigation
The area lying in t h e Zambezi d r a i n a g e basin i s divided i n t o t h r e e geographic a l regions: (a) t h e high veld, t h e highest extensive s u r f a c e which l i e s l a r g e l y
between 1 2 0 0 and 1 5 0 0 m e t r e s , forms t h e watershed between t h e Zambezi r i v e r
and t h e n o r t h . (b) In t h e western p a r t of t h e Zambezi basin t h e middle veld forms a
v e r y e x t e n s i v e plateau. ( c ) The Zambezi low veld is more limited a n d i s mostly cutoff from t h e rest of t h e country by s t e e p e s c a r p m e n t s a n d rugged t e r r a i n . The
main c r o p s are maize a n d Virginia tobacco. F o r e s t s are important, especially in
t h e n o r t h w e s t e r n q u a r t e r with extensive r e s e r v e s of indigenous hardwoods
c o v e r i n g some 9 5 0 0 km2, a n d in t h e e a s t e r n highlands t h e r e are a b o u t 650 km2 of
pines.
3. Water Quality
A w a t e r quality s u r v e y of t h e Zambezi r i v e r system i s not y e t v e r y developed
(UNEP 1986b), mainly d u e to t h e s h o r t a g e of manpower a n d equipment.
Yet,
hydro-chemistry of t h e middle a n d lower Zambezi i s relatively well documented. A
s u r v e y h a s been made principally of t h e main t r i b u t a r i e s of t h e Zambezi r i v e r , and
t h e main impounded zones. Thus, Coche (1968), and Balon and Coche (1974) have
detailed t h e physic-chemistry
of Lake Kariba, while McLachlan (1970a), King and
Lee (1974)and Bowmaker (1976)have focused on t r i b u t a r i e s entering Lake Kariba.
Hall et aL. (1976;1977) have described t h e physico-chemical s t a t u s of t h e lower
Zambezi p r i o r to and during t h e closure of t h e Cahora Bassa dam and its main t r i butaries. Hydro-chemical and hydrobiological surveys have been done also on t h e
Kafue f l a t s (Carey 1971; S a l t e r 1979; 1985). S o m e information i s available on
smaller impoundments like t h e McIlwaine lake on t h e Hunyani r i v e r (Marshall and
Falconer 1973a,b).
N o major pollution problems have been r e p o r t e d on t h e Zambezi drainage sys-
t e m (UNEP 1986b), e x c e p t at some specific s i t e s such as t h e copperbelt in Zambia,
Lusaka and H a r a r e areas, and some o t h e r major towns from which domestic and
industrial effluents could o c c u r . The regulation of t h e middle and lower Zambezi
and t h e i r transformation into a "reservoir r i v e r " might a g g r a v a t e t h e water quality problems, like eutrophication due to f e r t i l i z e r leaching. Although soil fertilization i s not developed, as shown in Table 19, t h e situation may change if l a r g e
scale irrigation development o c c u r , and also because development of agricultural
programmes i s one of t h e main goals of t h e r i p a r i a n countries. Another problem
t h a t could a r i s e soon i s sedimentation in man-made lakes due to soil erosion in
deforested areas (see Section 3). Presently, t h e water quality of t h e middle and
lower Zambezi depends mainly on processes t h a t o c c u r in t h e impoundments. Water
quality d a t a will b e given in Chapter 3 on impoundments.
TrrbLe 19. Fertilization rates in countries sharing t h e Zambezi drainage basin (FA0
Fertilizer Yearbooks 1981,1986).
Angola
Botswana
Malawi
Mozambique
Namibia
Tanzania
Zambia
Zimbabwe
1970
1976
1980
0.9
3.8
1.7
ND
1.7
4.1
23.1
2.1
0.7
0.4
8.3
3.5
2.2
1.5
ND
ND
2.6
2.8
7.5
7.9
24.2 27.8
1970
1984
1
1
0.4 0.27
5.5 0.27
2.412.18
ND
ND
3.0 1 0.31
8.510.83
30.3 15.85
I
1976
1980
1984
0.3
0.31 0.17
0.31 0.17
2.18 1.35
ND
ND
0.96 0.39
1.57 1.27
7.55 6.37
0.13
0.13
0.61
ND
0.44
1.40
7.24
Another problem r e l a t e d to t h e development of a g r i c u l t u r e is t h e use of pesticides, especially DDT to e r a d i c a t e tsetse flies which are a sine qua none condition
b e f o r e settlement c a n begin in t h e s e infested areas. DDT i s a l s o used for t h e e r a d ication of Anopheles flies, c a u s e of malaria. The u s e of pesticides i s dangerous
because of concentration in t h e food chain. Endosulfane and o t h e r "soft" pesticides have been t e s t e d as substitutes f o r DDT.
4. Energy Planning
The S o u t h e r n African Development Coordination Conference (SADCC) w a s established in April 1980 by a declaration of t h e governments of t h e nine independent
states of s o u t h e r n Africa, namely Angola, Botswana, Lesotho, Malawi, Mozambique,
Swaziland, Tanzania, Zambia and Zimbabwe. The objectives of t h e regional energy
policy h a v e been summarized by Bhagavan (1985) as follows:
-
"to r e s t r i c t t h e u s e of petroleum products solely t o applications where
a l t e r n a t i v e r e s o u r c e s cannot b e envisaged
- t o develop regional electrification and extend i t to t h e t r a n s p o r t and a g r i c u l t u r a l s e c t o r s , to exploit t h e vast h y d r o e l e c t r i c r e s o u r c e s of t h e region in
o r d e r to a c h i e v e this, and a l s o t o make use of small h y d r o e l e c t r i c power
plants throughout t h e r u r a l areas
- to
promote t h e interconnection of t h e national g r i d system t o e n s u r e t h a t
production and distribution capacity i s utilized more efficiently among t h e
various states in t h e region
- to develop
prospecting and exploitation of fossil fuel deposits, such as oil,
n a t u r a l g a s and coal
- to develop new technologies f o r t h e utilization of s o l a r energy, biomass and
o t h e r renewable e n e r g y s o u r c e s and t o make them available t o t h e r u r a l
areas
- to promote r e s e a r c h and development in renewable energy technologies at
t h e regional level
- to promote regional programmes of
and utilization of wood."
r e f o r e s t a t i o n and efficient exploitation
This review of objectives offers a c l e a r idea of what is at s t a k e on t h e Zambezi drainage basin since t h e signing of t h e declaration by seven of t h e eight
riparian countries of t h e Zambezi r i v e r , and i t is assumed by both SADCC and t h e
South West African People's Organisation (SWAPO) t h a t a n independent Namibia
would join SADCC.
4.1. Fuelwood
Fuelwood i s t h e primary s o u r c e of energy f o r all r u r a l households and a l a r g e
p a r t of t h e urban households of t h e r i p a r i a n countries of t h e Zambezi r i v e r . Table
20 gives t h e evolution of wood use as a s o u r c e of energy in t h e Zambezi r i p a r i a n
countries.
Table 20. Evolution of woodstock in 103m3 used as energy s o u r c e in t h e Zambezi
r i p a r i a n countries (FA0 Forest Products 1975, 1986).
Angola
Botswana
Malawi
Mozambique
Namibia
Tanzania
Zambia
Zimbabwe
Fuelwood plays a dominant r o l e in t h e curing of tobacco and tea, t h e major
cash c r o p s of t h e region. For instance, Bhagavan (1985) r e p o r t e d t h a t in Malawi
and Zimbabwe, tobacco and tea curing accounts for about 40 p e r c e n t of t h e total
firewood consumption. A s a n example, Hosier (1986) gives t h e energy consumption
in Zimbabwe (Figure 17) with t h e actual end-use and projected requirements for
t h e y e a r 2000. I t a p p e a r s t h a t even in Zimbabwe with abundant supply of coal and
electricity, wood r e s o u r c e s will continue t o b e essential for most of t h e foreseea b l e future. This poses a major problem due to t h e increasing scarcity of fuelwood
supplies in all t h e Zambezi r i p a r i a n countries. Due to this demand for fuelwood in
r u r a l and urban households a s well as clearing land f o r a g r i c u l t u r e and timber
,
8o
1
End-use requirements in 1982
Fuelwood
Coal
Commercial
wood
L ~ q u ~fuel
d
Electrlclty
Rural
Industry
Households
Transport
Agriculture Urban
Other
Households
SECTOR
Projected end-use requirements
for 2002
Fuelwood
Coal
Commerc~al
wood
Liquid fuel
Electr~clty
Rural
Industry
Households
Transport
Agriculture Urban
Other
Households
Figure 17. Energy consumption in Zimbabwe (Hosier, 1986).
t r a d e , r a p i d deforestation i s o c c u r r i n g on t h e Zambezi d r a i n a g e basin, contributing t o soil erosion as well as t o changes in runoff from such disturbed areas. Thus,
Mumeka (1986)h a s shown t h a t deforestation led t o a n i n c r e a s e in streamflow, and a
change in t h e flood h y d r o g r a p h of t h e Kafue headwaters. Elwell (1978;1984)and
Elwell a n d Stocking (1982)h a v e developed models to p r e d i c t soil losses by a r a b l e
lands in Zimbabwe. Such estimations of soil loss are v e r y important in determining
t h e impact of sediment t r a n s p o r t e d by r i v e r s on r e s e r v o i r life of man-made lakes
in t h e Zambezi basin (Bolton 1984;Kabell 1984;P i t t a n d Thompson 1984;White and
Bettess 1984).
4.2. Hydroelectric schema and other energy sources
F i r s t , dealing with oil production, Angola i s t h e only Zambezi r i p a r i a n country
t h a t p r o d u c e s a n d e x p o r t s substantial quantity of oil. On t h e o t h e r hand, t h e r e are
massive deposits of coal in Botswana, Mozambique, Tanzania, Zambia and Zimbabwe.
Unfortunately. t h e d e c r e a s e in world demand f o r coal h i n d e r s p r i v a t e foreign
investment f o r developing coal r e s e r v e s f o r e x p o r t (Bhagavan 1985). Concerning
n a t u r a l gas, big r e s e r v e s h a v e been discovered only at two o f f s h o r e s i t e s in
Mozambique.
Actual and potential h y d r o e l e c t r i c power are abundant. T h e r e i s more t h a n
enough installed generation c a p a c i t y today t o meet t h e c u r r e n t demands of u r b a n
households, industries a n d t h e s e r v i c e sector. Table 21 gives t h e net capacity
installed in e l e c t r i c i t y generation plants, while Table 22 shows t h e utilization of
e l e c t r i c i t y according t o t h e t y p e of plant.
Table 21. Installed c a p a c i t y of e l e c t r i c i t y generating plants of t h e Zambezi r i p a r i a n c o u n t r i e s (UN Yearbooks 1972,1983,1987).
Thermal c a p a c i t y lo3 kw
Angola
Botswana
Malawi
Mozambique
Namibia
Tanzania
Zambia
Zimbabwe
Hydroelectricity c a p a c i t y lo3 kw
1961
1970
1980
1985
1961
1970
1980
1985
60
101
200
200
28
211
400
400
ND
ND
ND
ND
ND
ND
ND
ND
11
94
21
241
39
280
34
280
28
114
67
1520
126
1523
ND
ND
ND
ND
ND
ND
ND
30
241
502
94
184
487
70
190
487
180
191
906
1
65
ND
20
43
562
49
180
705
188
1538
705
260
1538
633
Table 22. Utilization of installed e l e c t r i c i t y generating c a p a c i t y according to t h e
t y p e of plant and c o u n t r y .
1
Angola
1
Botswana
Malawi
1
I
Mozambique
1
Namibia
Tanzania
Zambia
Zimbabwe
1
Thermal KWH p e r KW
1
Hydroelectricity Kwh p e r KW
1961
1970
1980
1985
1961
1970
1980
1985
517
ND
3000
1383
ND
ND
1672
1147
1228
ND
609
1711
ND
1851
970
2388
2000
ND
769
1607
ND
2571
526
1094
2275
ND
771
1464
5357
ND
2000
1846
2464
ND
5038
2353
ND
6224
910
7443
2750
ND
6030
8914
ND
2819
5722
5685
3338
ND
3775
1008
ND
2365
6538
5355
183
1051
I
1
1
6186
3925
Nowadays, even if utilization of t h e existing e l e c t r i c i t y c a p a c i t y scheme i s
below t h e potential, t h e supply of e l e c t r i c i t y comfortably e x c e e d s demand in t h e
Zambezi r i p a r i a n c o u n t r i e s . Bhagavan (1985)underlined t h a t "demand h e r e means
ability of t h e end-use customer to pay t h e a s k e d f o r p r i c e which effectively l e a v e s
o u t a b o u t 70 to 90 p e r c e n t of t h e population, who at p r e s e n t d o not h a v e t h i s ability to pay." If a t t e m p t s were made to m e e t t h i s l a r g e r need, t h e p r e s e n t supply
would
b e clearly
inadequate.
L a r g e s t installed
and
potential capacity
in
h y d r o e l e c t r i c i t y of t h e Zambezi d r a i n a g e basin i s based mainly on t h e Zambezi
r i v e r itself and o n t h e Kafue r i v e r (Table 23).
Table 23. H y d r o e l e c t r i c power plants on t h e Zambezi and t h e Kafue r i v e r s (Bolton
1983).
Power P l a n t
Victoria Falls
Kariba
C a h o r a Bassa
Kafue G o r g e
Installed
capacity
in Mw
108
Provision
capacity
in Mw
5. Main Issues
From t h e g e n e r a l f e a t u r e s of t h e Zambezi r i v e r system a n d i t s d r a i n a g e basin,
i t a p p e a r s t h a t two main issues must b e f a c e d from t h e water management point of
view: ( a ) t h e soil e r o s i o n problem due t o intensive deforestation, a n d (b) reserv o i r o p e r a t i o n s in o r d e r t o optimize t h e i r h y d r o e l e c t r i c power generation and
planned secondary uses. In f a c t , t h e s e two main challenges are interconnected in
t h a t t h e erosion p r o c e s s entails a n i n c r e a s e in s i l t deposition in r e s e r v o i r s diminishing t h e i r r e a l s t o r a g e capacity.
The main man-made l a k e s on t h e Zambezi
d r a i n a g e basin l i e in t h e middle s t r e t c h of t h e Zambezi r i v e r (Kariba, Cahora
Bassa) and on t h e Kafue r i v e r (Figure 18). R e s e r v o i r o p e r a t i o n along t h e Zambezi
i s interconnected. F o r t h e s a k e of presentation, however, t h e following c h a p t e r
will d e a l with e a c h of t h e s e impoundments s e p a r a t e l y . Constraints imposed by
o p e r a t i o n of upstream dams will a l s o b e considered.
ItezhlTezhi
Kafue Swamps
Kafue
Gorge
+
,.--.\
-
I
Kariba
Ma na
Pools
Marromeu
/ - %
A
Cahor:
Bassa
I
I
Ocean
Figure 18. Schematic r e p r e s e n t a t i o n of impoundments on t h e middle Zambezi (Pinay et &., 1!388).
Chapter ILI: Impoundments on the Zarnbezi River System
1. Lntroduction
I t h a s a l r e a d y b e e n mentioned t h a t a b o u t 5 7 0 km of t h e Zambezi r i v e r are now
r e g u l a t e d by man-made l a k e s , r e p r e s e n t i n g 6 5 p e r c e n t of t h e middle Zambezi
s t r e t c h . Although t h e m o r e visible c o n s e q u e n c e of dam c r e a t i o n i s flow c o n t r o l of
t h e r i v e r , o n e must k e e p in mind t h a t i t a l s o l e a d s to development of d r a i n a g e
b a s i n - r e s e r v o i r i n t e r a c t i o n s (Figure 1 9 ) . S i n c e t h e s e i n t e r a c t i o n s are initiated by
r e s e r v o i r c o n s t r u c t i o n , development follows as a m a t t e r of c o u r s e , causing l a r g e
s c a l e environmental d i s t u r b a n c e s . Thus, i t i s e s s e n t i a l to c o n s i d e r t h e r e l a t i o n s h i p s between r e s e r v o i r a n d i t s d r a i n a g e basin. Although t h e s e man-made l a k e s
w e r e c o n s t r u c t e d f o r t h e s o l e p u r p o s e of h y d r o - e l e c t r i c p o w e r g e n e r a t i o n ,
management of t h e s e impoundments h a v e p o s e d f a r too many ecological p r o b l e m s
b o t h in r e s e r v o i r s a n d in t h e r i v e r downstream (Begg 1973; Davies 1975a-b; DuToit.
1 9 8 2 ; S h e p p e 1985). F u r t h e r m o r e , now t h a t t h e l a k e s are t h e r e , i t i s a c h a l l e n g e
to t r y to u s e them as a r e s o u r c e f o r optimal biological p r o d u c t i o n (Table 2 4 ) . It.
a p p e a r s t h a t f o r e v e r y function, t h e guidelines f o r optimal u s e could b e a hind r a n c e to achieving o t h e r water management goals.
CMANGING L A N E
WATERSHED
Figure 19. D r a i n a g e b a s i n - r e s e r v o i r i n t e r a c t i o n s (Simons, 1979).
Thus, t h e following c h a p t e r will d e a l with d i f f e r e n t uses and t h e i r c o n s t r a i n t s
f o r t h e main impoundments c i t e d in Table 24.
Table 24. Main u p s t r e a m and downstream p u r p o s e s of t h e major man-made lakes i n
t h e Zambezi r i v e r system (Pinay e t a l . 1988).
---
Kariba
Upstream
purposes
Fisheries
Irrigation
Recreation
Animal
husbandry
Swamps
Main p u r p o s e
of dam
+
+
+
+
I
1
1
Power
generation
Kafue G o r g e
C a h o r a Hassa
+
+
+
+
+
I
I
Downstream
purposes
Dam o p e r a t i o n
Navigation
Irrigation
Animal
husbandry
Fisheries
Swamps
maintenance
Itezhi-Tezhi
1
I
---
+
Kafue
swamps
Water
storage
Power
generation
Power
generation
Mana pools
Marromeu
I
C a h o r a Bassa
Kafue G o r g e
+
+
Mana pools
Kafue
+
2. Kariba Impoundment
2.1. H i s t o r i c a l Aspects
Originally t h e K a r i b a p r o j e c t was conceived as a national p r o j e c t . I t was cons t r u c t e d in t h e h e a r t of t h e F e d e r a t i o n of Rhodesia and Nyasaland t o p r o v i d e a
functional r e i n f o r c e m e n t of t h e political s t r u c t u r e . A loan f r o m t h e World Bank in
1956, g u a r a n t e e d by t h e United Kingdom a n d t h e F e d e r a t i o n , was with r e s p e c t t o
t h e p u r p o s e of power p r o d u c t i o n by installing a h y d r o - e l e c t r i c plant having a maximum c a p a c i t y of 1200000 kw (Austin 1968). L a k e K a r i b a was completed in 1 9 5 9 ,
inundating 5250 km2 of t h e f o r m e r Gwembe valley o n t h e middle Zambezi. Thc:
scheme was vital to t h e development of t h e F e d e r a l economy, p a r t i c u l a r l y to t h e
expanding c o p p e r mining i n d u s t r y in N o r t h e r n Rhodesia (now Zambia). Justifica-
tion f o r t h e s c h e m e w a s p a r t l y t h e f a c t t h a t i t would d e c r e a s e t h e c o p p e r b e l t ' s
d e p e n d e n c e upon Rhodesian c o a l . The F e d e r a t i o n of Rhodesia a n d Nyasaland was
dissolved at t h e e n d of December 1 9 6 3 , a n d l a t e r on O c t o b e r 2 4 , 3.964, N o r t h e r n
Rhodesia became t h e independent state of Zambia. A t t h e b r e a k - u p of t h e F'ederation, t h e two c o u n t r i e s e x p r e s s e d t h e d e s i r e t h a t " t h e i n t e g r a t e d system f o r t h e
c o n t r o l of t h e g e n e r a t i o n of e l e c t r i c power a n d i t s transmission in t h e t e r r i t o r i e s
should c o n t i n u e t o b e o p e r a t e d a n d fully developed as a single system u n d e r t h e
joint ownership of t h e government." (Austin 1968). The C e n t r a l African P o w e r Corp o r a t i o n was e s t a b l i s h e d in 1 9 6 3 in o r d e r to allow Zambia a n d S o u t h e r n Rhodesia
t h a t became Zimbabwe in 1980, t o s h a r e equally t h e a v a i l a b l e g e n e r a t i n g o u t p u t
f r o m t h e K a r i b a complex. Thus, a n i n t e r c o n n e c t e d system between t h e two count r i e s h a s b e e n developed (Figure 20).
2.2. M a i n Features o f M a n - M a d e Lake Kariba
T h e Zambezi r i v e r c a t c h m e n t a b o v e t h e K a r i b a dam i s composed of t h r e e main
g e o g r a p h i c a l e n t i t i e s (Figure 21): (a) N o r t h e r n Highlands, a b e l t of high ground:;
between 1 0 0 0 a n d 2 0 0 0 m e t r e s c o v e r i n g 220 670 km2; (b) C e n t r a l P l a i n s , a r e l a tively f l a t p l a t e a u between 1 0 0 0 a n d 1500 m e t r e s c h a r a c t e r i z e d b y l a r g e swampy
areas ( B a r o t s e a n d Chobe swamps), with a n area of 2 8 6 3 7 0 km2; ( c ) Rhodesian
Highlands comprising of a peneplain lying between 650 a n d 1 3 0 0 m e t r e s c o v e r i n g
1 5 6 1 8 0 km2. Total area c o v e r s a b o u t 6 6 3 8 2 0 km2, constituting m o r e t h a n a t h i r d
of t h e t o t a l d r a i n a g e basin of t h e Zambezi. L a k e K a r i b a i s s i t u a t e d on t h e Zambezi
at a n a l t i t u d e of 4 8 5 m e t r e s a b o v e sea-level. I t w a s completed in 1 9 5 9 when a concrete a r c h dam r i s i n g 1 2 8 m a b o v e t h e r i v e r b e d a n d measuring 5 8 0 m e t r e s long
w a s closed. I t i s s i t u a t e d at t h e b o u n d a r y between Zambia a n d Zimbabwe. L a k e
K a r i b a (Figure 22) i s divided i n t o f i v e b a s i n s defined by n a r r o w c h a i n s of islands
a n d b e l t s of shallow water. G e n e r a l c h a r a c t e r i s t i c s of t h e r e s e r v o i r itself are
given in Table 2 5 a n d in Annex I.
,
I
110
R A Y Y ( S S L * LINES
O T M R ~ R U I Y . 5 Y Z 1W
I
I
--
L
1
-
,
-
I
R E P U B L I C OF
Figure 20. I n t e r c o n n e c t e d system between Zambia and Zimbabwe (CAPC 19f14).
I
Figure 21. Drainage basin of t h e Zambezi r i v e r upstream from t h e Kariba D a ~ n
(Balon and Coche, 1974).
Table 25. General c h a r a c t e r i s t i c s of K a r i b a r e s e r v o i r (Jackson and Davies 1976).
Feature
Catchment
Geographical position:
Longitude
Latitude
Direction of main a x i s
Direction of predominant wind
Number of basins
Height of wall
Maximum d e p t h
Mean d e p t h
Maximum drawdown
Maximum length
G r e a t e s t width
Total a r e a at c a p a c i t y
Maximum floodgate d i s c h a r g e
Power c a p a c i t y p e r t u r b i n e
Total power output of dam
Actual filling time
Impounded water mass
Infestant a q u a t i c macrophytes
p r e s e n t in t h e system
Kariba
663,820
m
m
m
m
km
km
3km'
m sec-1
MW
MW
years
26'40'E-2g03'E
16"2E1S-1E06'S
SW-NE
SE-NW
5
128
120
29.5
14
300-320
40
5 250
6 500
100
1200
-
m3
1. S a l v i n i a molesta
2. P i s t i a s t r a t i o t e s
2.3. Hydrology
The Zambezi i s t h e major r i v e r flowing into Lake Kariba and provides between
7 0 and 8 0 p e r c e n t of i t s water. The remainder is supplied by o t h e r t r i b u t a r i e s
and d i r e c t rainfall. Average inflow into t h e Kariba l a k e from t h e Zambezi between
1925 and 1959 was 1133 m3s-I (Balon and Coche 1974), while t h e t o t a l input from
o t h e r t r i b u t a r i e s was 319 m3s-l. On t h e basis of t h e i r individual catchment a r e a s ,
t h e main secondary r i v e r s are t h e Gwaai, Sanyati and Sengwa (Table 26, Figure
22). Altogether t h e s e r i v e r s situated in Zimbabwe d r a i n 63.6 p e r c e n t of t h e l a k e
catchment area.
ZIMBABWE
Figure 22. Man-made Lake Kariba and i t s main t r i b u t a r i e s (adapted from Marshall
a n d J u n o r , 1983).
Table 26. D r a i n a g e basin areas of t h e main s e c o n d a r y t r i b u t a r i e s of L a k e Kar.it)i:
K a r i b a Basins
North S h o r e
I
Drainage basin
km2
-
1
South S h o r e
I
Tributaries
Tributaries
Mlibizi (1)
Gwaai
1
D r a i n a g e basin
krn2
I
47 1 4 0
I
Chimene
Binga (2)
2 290
850
Zongwe
Chezya
Sengwa (3)
2 274
Lufna
Bumi (4)
2 567
-
A[
----- -- --
Mwenda
Zengwa
27"
5100
One of t h e c h a r a c t e r i s t i c s of t h e s e c o n d a r y t r i b u t a r i e s i s t h e i r e r r a t i c :
discharge.
For
instance,
absolute
minimum
and
maximum
average
annual
d i s c h a r g e s were 7 5 m3.s-I in 1946/47 a n d 8 5 2 rn3.s-I r e s p e c t i v e l y (Ralon and Coche
1974). T h e r e f o r e , o v e r a l l a v e r a g e d i s c h a r g e (318 m3.s-l) h a s n o t c h a n g e d s i n c e
t h e c r e a t i o n of L a k e K a r i b a , although t h e a v e r a g e Zambezi inflow h a s increi~::ud
f r o m 1 . 9 9 9 m3.s-I between 1925-1966 p e r i o d t o 1 . 5 1 7 m3.s-I between 39C)9-1~9titIb
Concerning t h e hydrological c y c l e , t h e s e c o n d a r y t r i b u t a r i e s featur.e a main flood
s e a s o n between J a n u a r y a n d March ( a b o u t 5 6 p e r c e n t of t h e a n n u a l d i s c h a r g e ) ,
a n d c o n s t i t u t e t h e Gumbura floods c i t e d previously ( C h a p t e r I , S e c t i o n 3.2). T h e
Zambezi flood (Murorwe) o c c u r s between March a n d May ( F i g u r e 1 1 ) .
2.4. R e s e r v o i r O p e r a t i o n
The dam was c l o s e d o n D e c e m b e r 2, 1958, when t h e l a k e s t a r t e d to fill f r o m i t s
r i v e r - b e d b a s e of 3 9 1 m a b o v e sea level. The filling p h a s e finished in 1 9 6 3 . During
t h e f i r s t p h a s e p e r i o d , w a t e r l e v e l f l u c t u a t i o n s w e r e v e r y i m p o r t a n t ( F i g u r e 23).
Water l e v e l s rose f r o m 420 m a b o v e sea l e v e l in 1959-1960 e x t r e m e l y r a p i d l y (6
m e t r e s e v e r y 24 h o u r s ) . Until t h e e i g h t m e t r e s d r o p in 1964, conditions had b e e n
improving. Considering t h e K a r i b a l a k e ' s b a t h i m e t r y (Figure 2 4 ) t h e s e drawdowns
e n t a i l i m p o r t a n t v a r i a t i o n s o n t h e waterlogged area. During t h i s filling p e r i o d ,
3 -1
d i s c h a r g e s from K a r i b a dam were much lower (annual a v e r a g e 258 m . s ) , com3 -1
p a r e d to t h e n a t u r a l flow of t h e Zambezi r i v e r at K a r i b a s t a t i o n ( 1 423 m . s ).
Table 2 7 g i v e s t h e monthly flows f r o m t h e K a r i b a dam d u r i n g t h e filling p h a s e .
Figure 29. L a k e l e v e l s on t h e K a r i b a ( m e t r e s a.s.1.) r e c o r d e d o v e r t h e per,ioc!
1961-71 (Begg, 1973).
Table 27. L a k e K a r i b a monthly d i s c h a r g e s in c u b i c m e t r e s p e r s e c o n d (Balorl arrd
Coche 1974).
- .
Year
Oct
Nov
Dec
Jan
Feb
Mar
1958-59
1959-60
1960-61
1961-62
-.
472
283
330
378
472
33
283
142
283
330
3 7 8-- 3 7 8
24
142
330
378
14
142
283
330
19
19
19
24
194
142
142
189
1 8 9 236
330
330
330
3 3 0 330
3 7--8 - 3 7 8 -- 37 8- -425
425
- ..-
Apr
May
Jun
Jul
Aug
Sept
.--.-
-
288
283
378
425
-
270
283
378
37H
-
S i n c e t h e end of t h e filling p h a s e , r e s e r v o i r o p e r a t i o n h a s managed t o provic!~:
t h e b e s t h y d r o p o w e r p r o d u c t i o n . T h e Zambezi flow o r d i s c h a r g e below t h e K a r i b a
dam c o n s i s t s of two components whose i m p o r t a n c e v a r i e s g r e a t l y a c c o r d i n g t o t h e
e n g i n e e r i n g n e e d s d u r i n g t h e y e a r : ( a ) t h e t u r b i n e flow r e l e a s e d t h r o u g h t h e t a i l
races i s closely r e l a t e d to t h e e l e c t r i c i t y power r e q u i r e m e n t s . T h e c e n t r e s of t h e
l a k e water i n t a k e s are s i t u a t e d at a b o u t 462.5 a n d 447.5 m.a.s.1. r e s p e c t i v e l y . At
a n a v e r a g e o p e r a t i n g water l e v e l of t h e r e s e r v o i r (485 m.a.s.l.), t h e w a t c ? r i s
t h e r e f o r e drawn f r o m a d e p t h of at l e a s t 2 0 m e t r e s ; (b) t h e s p i l l a g e flow is
r e l e a s e d t h r o u g h o n e or m o r e of t h e six s l u i c e g a t e s built i n t o t h e dam (457-466 m)
f o r controlling t h e l a k e water level (Balon a n d C o c h e 1974). Maximum floodgate
3
-1
d i s c h a r g e i s a b o u t 6 5 0 0 m .s
b u t v a r i e s depending on t h e hydrological condi-
tions. F o r i n s t a n c e , T a b l e 28 g i v e s t h e number of d a y s of f l o o d g a t e d i s c h a r p e
between 1 9 6 1 a n d 1 9 7 8 .
Figure 24. B a t h y m e t r y o f L a k e K a r i b a ( B a l o n a n d C o c h e , 1974).
l I b M s 227. Floodgate discharge at Kariba dam in days p e r month between 1961-1978
(Guy 1981).
Year
Jan
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
4
6
22
31
31
17
31
20
31
8
5
Feb
Mar
28
29
12
1
9
29
28
28
28
13
31
4
1
31
31
31
31
31
15
28
5
28
31
31
28
31
Apr
May
29
30
1
11
12
30
27
30
30
30
30
27
30
25
30
Month
Jun Jul
3
11
20
Aug
Sept
31
31
12
24
31
30
Oct
Nov
Dec
17
30
31
21
16
31
31
2
23
1
31
1
31
31
31
30
10
30
31
30
15
29
31
2
Energy demand constitutes t h e main p a r a m e t e r regulating t h e turbine
discharge. Current monthly turbine flow r e l e a s e s are almost t h e same whatever
t h e period considered (Figure 25). Evaporation i s a n o t h e r kind of output from t h e
lake. Since t h e creation of Lake Kariba, increased evaporation and increased
rainfall o v e r t h e l a k e have affected t h e downstream flow. According to Dutoit
(1982), between 1975/76 and 1979/80,about 7 p e r c e n t of t h e total annual inflow
into Lake Kariba consisted of rainfall, while 14 p e r c e n t of t h e t d a l annual inflow
was lost through evaporation.
In terms of total input and output from t h e Kariba r e s e r v o i r , i t seems, as mentioned previously (Chapter I, Section 3.1) t h a t t h e monthly downstream flow regime
was much t h e same as i t would have been without the Kariba dam. On t h e o t h e r
hand, Guy (1981) showed from inflow and outflow r e c o r d s (1966-1978 Figure 26)
t h a t seasonal differences have become more pronounced, with w e t seasons being
"wetter" and d r y seasons "drier". This remark i s in contradiction with t h e r e s u l t s
of Attwell (1970) where r e s e r v o i r operation of the Kariba tends to stabilize t h e
flow. DuToit (1982) r e p o r t s t h a t t h e mean annual flow of t h e u p p e r Zambezi w a s
Figure 25.T u r b i n e flow r e l e a s e s of t h e K a r i b a Darn. July 1983-June 1 9 H 4 ( ( : & l J C ,
1984).
42.5. 109m3 with a minimum of 19.2.109m3 in 1958/59.
F u r t h e r m o r e , extrernt:
monthly flows w e r e 2 3 . 1 109m3 in March 1958 a n d 0 . 3 7 . 1 0 ~ min~ November 1924.
This c o n s t i t u t e s a r a t i o of 6 2 : l . Thus, t h e a n n u a l a n d s e a s o n a l v a r i a t i o n s in t h e
flow of t h e Zambezi are of m a j o r s i g n i f i c a n c e to t h e e c o l o g i c a l dynamics of t h e
r i v e r downstream f r o m t h e dam.
l h e rnonthl! d ~ l l ~ ~ r c n c e ~ h c t w c c n t h c a r n ooul u
na
t t c r f l o u i n g i n t o a n d out ol I aLc R a r ~ h a1he
Ilnlt, arc r n ~ l l ~ a r d * l r nlU')
' x The u p p c r f i ~ u r ci n d l c a ~ c s ~ hnormal
c
p a l ~ c r n o l f l o ulor thc Zarnhcr~R ~ \ e r
~ J \ L .on
c Ithc ~ n l l o ufipurc\ lor I ~ k K
c a r ~ h aOctohcr
.
(thc *tart o l the h!drolog~cal !cart I \ lndlcatcd nn
~ h ch o r ~ / u n t a laxl\ The d a ~ aarc lor the pcrlud Octobcr 1%
to July IY?&
Figure 26. Water balance in t h e Kariba r e s e r v o i r (Guy, 1901).
2.5. Water Level F l u c t u a t i o n s
Routine in t h e c o u r s e of management of a h y d r o e l e c t r i c power instal la ti or^ is
t h e determination of d i s c h a r g e rates t o m e e t power demand and flood c o n t r o l .
These activities, in t u r n , a f f e c t t h e water levels of t h e l a k e (Magadza 1906). Thus,
t h e l a k e i s c h a r a c t e r i z e d by l a r g e fluctuations in t h e water level with a n annual
amplitude of a r o u n d t h r e e m e t r e s and long-term fluctuations t o e x t r e m e s of more
t h a n eleven metres (Ramberg et al. 1907). The promising e f f e c t s of drying and
wetting soils are quite well known, a p r o c e s s t h a t a c c e l e r a t e s t h e decomposition of
o r g a n i c s resulting in n u t r i e n t r e l e a s e (Chauvet 1987; F a b r e (forthcoming); Pinay
a n d Decamps 1988). It i s likely t h a t t h e p r o c e s s of drying and wetting i s largely
responsible f o r t h e important flushes of g r a s s in t h e Kariba l a k e s h o r e , t h e inundation of which led to t h e r e l e a s e of l a r g e quantities of materials into t h e water. A
s h o r e line exposed t o drawdowns are c h a r a c t e r i s t i c s of a n ecotone: "the zone of
transition between a d j a c e n t ecological systems, having a set of c h a r a c t e r i s t i c s
uniquely defined by s p a c e a n d time s c a l e s and by t h e s t r e n g t h of t h e interactions
between a d j a c e n t ecological systems" (Clements 1905; Holland 1907). The margin
of t r o p i c a l man-made l a k e s a c t as man-made floodplains: l a k e level fluctuations
a d d t o t h e o r g a n i c a n d mineral n u t r i e n t pools of both t h e t e r r e s t r i a l and aquatic:
components of t h e ecosystem through drowning during advancing and standing
Figure 27. Interactions between terrestrial and aquatic ecosystems during waterlevel fluctuations (Davies, 1986).
vegetation, and animal material during retreat. In addition, migratory terrestrial
herbivorous interactions take place in this ecotone (McLachlan 1974; Davies 1986)
(Figure 27). These interactions a r e beneficial f o r game pastures and lake productivity.
Thus, n u t r i e n t e n r i c h m e n t i s a positive e f f e c t o n submerged p l a n t s . F i g u r e 2 R
g i v e s a n example of t h e d e p t h d i s t r i b u t i o n of a n a q u a t i c p l a n t community. This
e n t i r e community f o r m s a n i m p o r t a n t s o u r c e of c o n s u m e r food as well as a prott:ct i v e canopy f o r i n v e r t e b r a t e s a n d fish (McLachlan 1969; 1 9 7 0 a , b ) . If t h e water
level f l u c t u a t i o n s h a v e a p o s i t i v e r e c i p r o c a l e f f e c t f o r t e r r e s t r i a l as well as f o r
a q u a t i c l i f e , i t i s i m p o r t a n t t h a t t h e p a t t e r n of water level v a r i a t i o n in Lake
K a r i b a is c o n t r o l l e d to r e s e m b l e t h e n a t u r a l c y c l e of t h e Zambezi r i v e r without
Figure 28. Example of d i s t r i b u t i o n p a t t e r n of some of t h e m a j o r a q u a t i c p l a n t s on
t h e K a r i b a s h o r e l i n e (McLachlan, 1 9 6 9 ) .
a r t i f i c i a l floods o u t of s e a s o n . F o r i n s t a n c e , Bowmaker (1973) recommended t h a t
t h e half-cycle f l u c t u a t i o n in l a k e l e v e l s should n o t e x c e e d 2 m e t r e s , a n d t h a t t h e
rate of c h a n g e should b e l e s s t h a n 0 . 6 metre/month.
2.6. Water Chemistry
S i n c e t h e K a r i b a r e s e r v o i r h a s r e a c h e d i t s post-filling p h a s e , t h e Zambezi
p r o v i d e s a b o u t 7 0 p e r c e n t of inflow water a n d c o n t r i b u t e s most of t h e input of dissolved a n d s u s p e n d e d m a t e r i a l s in t h e l a k e . T h e r e f o r e , t h e Zambezi u p s t r e a m from
t h e K a r i b a h a s a r e l a t i v e l y l o w n u t r i e n t l e v e l c o m p a r e d t o o t h e r r i v e r s (Mitchell
1973). The e l e c t r i c conductivity of water h a s b e e n found to b e a useful index f o r
estimating t h e d e g r e e of mineralization of w a t e r s . In t h e case of t h e K a r i b a reserv o i r , s t r o n g r e l a t i o n s h i p s e x i s t between conductivity o n t h e o n e hand, a n d total
solids c o n t e n t , salinity, total ionic c o n c e n t r a t i o n s a n d t o t a l alkalinity o n t h e other(Balon a n d Coche 1974). Thus, t h e mineral c o n t e n t i n c r e a s e s from Basin 1 to Basirr
5. The proximity to t h e Zambezi d i c t a t e s t h e physico-chemical c h a r a c t e r i s t i c s of
Basins 1 a n d 2; b u t t h e s e r i v e r i n e f e a t u r e s c h a n g e to l a c u s t r i n e f e a t u r e s in t h e
o t h e r basins. S i n c e t h e c l o s u r e of t h e K a r i b a dam, f o u r p h a s e s of chemical evolution may b e distinguished in t h e most l a c u s t r i n e p a r t of t h e r e s e r v o i r on t h e b a s i s
of t h e p a t t e r n of v a r i a t i o n of t h e water c o n t e n t in dissolved s u b s t a n c e s (Figure
29).
L.5
.
1959
111
1963
1966
1969
1972
I
-'
500
-490
--o ----G---,o---O---t
0.---+ - - - - G - - - (
-480
Max water level
-470
-
-460
5
*
2
*
rn
P
rn
\
-450
t20rn~cromhos
Conducl~voly
.*.
h
.-C
U
-----A-
-440
'
8
--
-
7
-470
D
-410
9
V,
D
VI
-400
1.5
'"
- 390
.-0-
*-FILLING
-TRANSITION
-
STABILIZATION
l95l'l95Q'1l6O'l96l1~962'1961'1964'l965'1~66'l96I'1968'l96dl97O~I97i'l97~
1973
Figure 29. T h e p h a s e s of chemical evolution in S a n y a t i b a s i n of L a k e K a r l b a
(Balon a n d Coche, 1974).
P h a s e I c o r r e s p o n d s to t h e f i r s t y e a r of r a p i d flooding of t h e valley soils,
adding l a r g e q u a n t i t i e s of o r g a n i c a n d i n o r g a n i c m a t t e r to t h e solution. During
p h a s e 11, t h e rate of flooding d e c r e a s e s , a n d t h e newly flooded s o i l s are l e s s r i c h
t h a n t h o s e flooded e a r l i e r . P h a s e I11 s t a r t e d at t h e completion of t h e filling p h a s e
- l a c u s t r i n e conditions d e v e l o p a n d
salinity i n c r e a s e s . During P h a s e IV, chemical
c h a r a c t e r i s t i c s v a r y a c c o r d i n g to t h e s t o r a g e level of t h e r e s e r v o i r a n d within a
n a r r o w r a n g e . On t h e b a s i s of i t s t o t a l mineral c o n t e n t , t h e K a r i b a l a k e c o n t a i n s
w a t e r s of low t o t a l ionic c o n c e n t r a t i o n s (Balon a n d Coche 1974). T a b l e 2 9 g i v e s t h e
main chemical n a t u r e of t h e Zambezi r i v e r u p s t r e a m a n d downstream from t h e
r e s e r v o i r as well as in t h e l a k e .
Although t h e Zambezi r i v e r must b e c o n s i d e r e d as t h e m o s t i m p o r t a n t f a c t o r in
determining t h e l a k e ' s chemical c h a r a c t e r , p a r t i c u l a r l y f o r t h e two u p p e r basins,
t h e local i m p o r t a n c e of t h e o t h e r inflowing r i v e r s c a n n o t b e i g n o r e d . S e c o n d a r y
r i v e r s are i m p o r t a n t in terms of t h e chemical n a t u r e of t h e i r w a t e r s . Begg (1969)
Table 29. Main c h e m i c a I c h a r a c t e r i s t i c s o n t h e Zambezi r i v e r flowing t b r o u g t ~
K a r i b a l a k e (Davies 1986).
Variable
Units
Water t e m p e r a t u r e
pH
Dissolved oxygen
TransparencySecchi
Total a l k a l i n i t y
Chloride
Calcium h a r d n e s s
Total h a r d n e s s
Zambezl at
K a r i ba
i nflow
Station
Lake K a r l ba
Basin 5(')
Lake K a r i ba
0utflow2
Homothermy
e s t a b l i s h e s at
22m
8.4(0.44)
Colour
Conductivity
Ammonia
Nitrite
Nitrate
Orthophosphate
Silica
Sulphate
Magnesium
Sodi urn
Potassium
Calcium
Iron
Manganese
all et al. (1977)
(e)Calculated from Coche (1968) a n d Balon & Coche (1974).
(3)Calculated from Coche (1968) as mean of a n n u a l c y c l e f r o m t h e e a s t e r n s e c t l o n of Basin
4 ( t h e e q u i v a l e n t of Basln 5 of Begg (1970), c l o s e to t h e dam wall).
(')calculated from Begg (1970).
S S u r f a c e measurements (Kariba).
B Bottom m e a s u r e m e n t s (Kari ba).
suggests t h a t Lake Kariba receives t h e major p a r t of i t s supply of dissolved salts
from secondary rivers. In f a c t they mostly influence t h e lake's chemical content in
t h e vicinity of t h e i r e n t r y (Balon and Coche 1974). The chemical contribution of
t h e Zimbabwean input waters is particularly important due t o t h e i r l a r g e drainage
basin (Table 26) (King and Lee 1974; Bowmaker 1976; King and Thomas 1985). The
Sanyati r i v e r , f o r instance, has g r e a t influence on t h e limnological characteristics
of Basin 5 at t h e e a s t e r n end of Lake Kariba.
Concerning t h e r e s e r v o i r itself, t h e establishment of a thermocline during t h e
w a r m season entails a separation of deep water from surface ones. This hypolimnion (below t h e thermocline) is r i c h e r in mineral content than t h e epilimnion.
Furthermore, mineralization processes t h a t o c c u r in t h e lower l a y e r s entail a
depletion in dissolved oxygen, which could, in t u r n , lead t o a solution of f e r r o u s
iron and hydrogen sulfide (H2S). These processes r e f l e c t oxido-reduction potentials in t h e hypolimnion. This w a s t h e case during filling and post filling phases.
Nowadays, reduced conditions still o c c u r until t h e water mixes in t h e cooler season. These reduced conditions hinder fish settlement in t h e hypolimnion. On t h e
o t h e r hand, when deoxygenated water is discharged into t h e r i v e r , e i t h e r by
floodgate spillage o r via t h e tailraces from t h e lowest turbines, t h e dissolved oxygen concentrations in t h e r i v e r downstream from t h e dam a r e liable t o drop.
2.7. Sediment Deposition
The area of t h e drainage basin of Lake Kariba is about 650.103km2. Of this,
4 0 . 1 0 ~ k m lies
~
upstream from t h e Victoria Falls. The Barotse plain and Chobe
swamps in t h e upper Zambezi catchment a c t as a sediment t r a p f o r virtually a l l t h e
sediment from this p a r t of t h e basin. Thus, t h e silt load of t h e r i v e r water as i t
r e a c h e s t h e lake is relatively light. Tributaries downstream from t h e Victoria
Falls drain t h e remaining 1 7 0 . 1 0 ~ k mof~ Lake Kariba's drainage basin; 1 4 0 . 1 0 ~ k m ~
lie in Zimbabwe, and 30.103km2 lie in Zambia. Finally, i t a p p e a r s t h a t t h e bulk of
t h e sediment deposited in Lake Kariba originates from t h e tributaries.
It is
estimated (Stocking and Elwell 1973; Bolton 1984) t h a t t h e sediment yields of t h e
drainage basin of Lake Kariba downstream from t h e Victoria Falls lies in t h e r a n g e
40-400 t/km2/year. Figure 30 gives as a n example, t h e potential erosion hazard in
t h e Zimbabwean p a r t of t h e Kariba drainage basin.
On t h e basis of t h e s e estimations, t h e mean annual input of sediment to Lake
Kariba lies in t h e r a n g e of 7 to 70.10% (Bolton 1984). In t h e case of Lake Kariba,
t h e long time scale involved suggests t h a t m o s t of t h e sediment will r e a c h a high
-
w
k4mt10l crmlm hazord
based on Scoch~ng and Elwell
Z Very l a w
Drolnopc basinr sludlcd
and Chthworh
by Ward
to
L B d o u ovcmgc to a n a g r
Figure 30. Potential erosion hazard in Zimbabwe (Stocking and Elwell, 1973; Bolton, 1984).
degree of consolidation, and Bolton (1984) assumes t h a t t h e annual rate of loss in
storage capacity of Lake Kariba lies between 7 and 70.106.m3. The dead storage
~,
o v e r 60 p e r cent of total
capacity of Lake Kariba is about 1 1 6 . 1 0 ~ mrepresenting
r e s e r v o i r capacity. A t t h e p r e s e n t rate of input, this storage would be filled in
1600-16000 years. Bolton (1984) concludes that t h e effect of sediment on t h e
operation of t h e project is small. Nevertheless, such siltation should o c c u r n e a r
secondary tributary inlets, and would b e appreciable at this small scale with
r e s p e c t to navigation o r fisheries.
2.8. Ecological Aspects
The formation of a lake provides a changed habitat f o r t h e development of a
new ecosystem. Immediately a f t e r t h e r i v e r is dammed, t h r e e phenomena a p p e a r
simultaneously: (a) t h e normal flow of t h e r i v e r is stopped, (b) t h e r e i s a continuous increase in t h e area of t h e new habitat as t h e lake fills up; (c) expanding lake
margins drown t h e surrounding land. Initially t h e resulting environment is initially
extremely unstable, but a f t e r a few y e a r s r e a c h e s a n "equilibrium".
The major
f a c t o r in t h e filling phase is t h e continuous increase in t h e size of t h e lake, and
t h e major input is derived from t h e newly inundated materials a t t h e advancing
shoreline. During this phase, t h e contributions of r i v e r s i s relatively minor.
Thus, t h e nutrients leached from t h e flooded t e r r e s t r i a l habitats and t h e rising
lake waters enable a n increased productivity. During t h e filling phase, t h e floating f e r n Sadvinia molests showed a n explosive growth, and with t h e help of t h e
available nutrients, this aquatic weed covered 22 p e r cent of t h e lake's s u r f a c e by
1962, but l a t e r (1970) declined to 15 p e r cent. Coverage declined again to about 1
p e r cent of t h e l a k e in 1980 (Marshall and Junor 1981). This well-known natural
phenomenon of increased production in newly c r e a t e d lakes is also noticeable in
fish production. When filling is completed (Figure 31) t h e extraneous input from
r i v e r s naturally remains t h e same, while t h e now almost s t a t i c shoreline i s relatively small (Mchchlan 1974).
Some possible pathwaya by m m s of which materials enter m d l a v e the lake emrystem
during the hlling p h u e (A) and the post-filling p h u e (B). T h e probable relative importance of
a& pathn-ay is indiated by the thicknar of the u r o w . Input from ( a ) afRuent river, (6) decomposition of flooded biota and leaching of mil, (c) eroding shore-liner, (4
decomporition of
aquatic biota. Uptake by (e) exploding animal m d plant populations. Lar
in effluent.
Position of thennaline is indiated by a broken line. Living t e r n t r i a l woodland, d m e d
f o m u , aquatic biota m d the bm wall arc shorn, in both tnnaccta.
Figure 31. Relative importance of input and output of materials during t h e filling
and post-filling phases of Kariba r e s e r v o i r (McLachlan, 1974).
Another important f e a t u r e of l a r g e r e s e r v o i r s , such as t h e Kariba, i s t h e
thermal stratification of water during t h e hot season from November to March.
Thus, a thermocline lies at about 25 metres below t h e s u r f a c e , and r a r e l y goes
below 35 metres (Coche 1968; McLachlan 1970a). The establishment of such a t h e r mocline s e p a r a t e s t h e oxygenated epilimnion and t h e cooler deoxygenated hypolimnion. A s stratification o c c u r s , a chemocline simultaneously develops at t h e
depth of maximum density change, coinciding with t h e thermocline and t h e
oxycline.
The natural fish population in t h e middle s t r e t c h of t h e Zambezi b e f o r e
impoundment is described as a p r o p e r t y of t h e fauna r e f e r r i n g largely t o t h e main-
stream where, since no "reservoir" swamps o c c u r (as on t h e u p p e r Zambezi), r i v e r
flow dwindles almost to a t r i c k l e between sandy pools during t h e d r y season (Bell
Cross 1972;Jackson 1986). From 156 species r e c o r d e d in t h e Zambezi r i v e r system, only 27 have been r e c o r d e d in t h e Gwembe valley b e f o r e i t w a s flooded by
Lake Kariba (Balon 1978). With t h e formation of Lake Kariba, a n area of 5 250 km2
of s t a t i c water w a s suddenly imposed on t h e fish. A s t h e rising waters gradually
moved to t h e floor of t h e Zambezi valley, t h e r e w a s m o r e s p a c e , more floods
o c c u r r e d each y e a r , and many species of fish flourished.
Balon quoted, f o r
instance, t h a t in less than t w o y e a r s t h e native species Brachyalestes imberi,
v e r y successful during t h e filling phase w a s entirely displaced by a n invader from
t h e u p p e r Zambezi: Atestes t a t e r a l i s more adapted to t h e new conditions. Disper-
sal of fish species according t o conditions existing in t h e five basins of t h e Lake
I
YEAR
P
i 32. Evolution of littoral and pelogic c a t c h e s in the Zimbabwean p a r t of
Lake Kariba (Ramberg nt ad.,1987).
Kariba i s dictated by a marked gradation from fluviatile to p a r t l y lacustrine, to
lacustrine conditions, when passing from Basin 1(Mlibizi) through Basin 5 (Sanyati)
(Begg 1974). Thus, t h e Cyprinidae family, r e p r e s e n t e d by Labeo congoro and Labeo
u l t i v e l i s in t h e commercial c a t c h is dominant in r i v e r i n e basins (1 and 2) and in
t h e vicinity of affluent r i v e r s while t h e Cichlidae family composed mainly of R l a pia mortimetf, ZUapcar rmdaLlC and' S s r g o c h r o m i s c o d r i n g t o n i h a s adapted to
t h e lacustrine d i t i o n s . F o r example, t h e b t a l l i t t o r a l c a t c h o n t h e Zimbabwean
side of Lake Kariba w a s at f t s highest in 1964 (Pigure 32), when 2 6 0 0 tons were
landed, a f t e r which it declined b between 800-1000 b n s p e r y e a r (Ramberg e t al.
1987) caused mainly by t h e development of t h e initial high leaching of nutrients
from t h e submerged land. A s a r e s u l t , a f t e r a s h a r p i n c r e a s e in prcxiuction i t went
down d u e to t h e f a c t t h a t a l l t h e dominant l i t t o r a l fish s p e c i e s were p r e s e n t in t h e
middle of t h e t r i b u t a r i e s of t h e Zambezi b e f o r e t h e l a k e w a s formed, and many of
them are typical r i v e r i n e species. Concerning t h e pelagic fishes, L i m n o t h r i s s a
mtodon w a s introduced from l a k e Tanganyika in 1968. This small sized clupeid sardine locally called "kapenta" propagated rapidly and commercial fishing s t a r t e d in
1974 on t h e Zimbabwean side. A t p r e s e n t , most of the c a t c h in Lake Kariba (some
8000 of 10000 tons in 1980), consists of t h e s a r d i n e L i r n n o f h r i s s a miodon. This is
p a r t l y b e c a u s e t h i s fish i s short-lived with a r a p i d breeding potential (reaching
c a t c h a b l e size 4-5 c m in length, a f t e r five to six months) and because i t i s situated
at t h e end of a s h o r t food-chain (phytoplankton, zooplankton, L. miodon), i t is a b l e
to respond to periods of t r a n s i e n t food abundance (Marshall ef a l . 1982). Thus, t h e
tigerfish ( h y d r o c y n u s v i f t a f u s ) h a s switched from a d i e t of l i t t o r a l fish to t h e
pelagic L. miodon. Prediction models on Lake Kariba's s a r d i n e yields are a b o v e
r e a l c a t c h e s (Machena a n d F a i r 1986). Total annual c a t c h in Lake Kariba w a s
a b o u t 24 000 tons in 1985, with 15000 tons f o r Zimbabwe r e p r e s e n t i n g 80 p e r c e n t
of i t s production), and 9000 t o n s f o r Zambia (Magadza 1986; Ramberg e t ul. 1987).
I t a p p e a r s t h a t fish production as a s o u r c e of protein f o r t h e population i s o n e of
t h e main t a r g e t s of r i p a r i a n countries, and especially f o r Zimbabwe (Kenmuir
1982).
Nevertheless, there are limihtions on fish production: (a) t h e f e e b l e primary
production, d u e to small input from t h e Z m b e z i ; (b) r a p i d t u r n o v e r (less than f o u r
y e a r s ) of t h e water in t h e r e s e n o i r which p r e v e n t s accumulation of energy; (c)
thermal stratification hindering n u t r i e n t exchange within w a t e r l a y e r s (Mtada
1987); (d) l a r g e drawdowns o u t of season which limit fish spawn. Another f e a t u r e
to b e t a k e n into account i s t h e significance of t r i b u t a r i e s in fish yields. Owing t o
/
l U T R l l N 1 LOAD
0
0
0
_
+
-........
_
_
_
_
.--
G
rib,,:
ar
......,~
,.r.rl
/
_ _ -_- _ _ - - _.-t,!m-l,F:
I...@,
F i i e 33. Evolution of Lake Kariba in t e r m s of ecological v a r i a b l e s (Balun,
1978).
t h e i r n u t r i e n t r i c h n e s s compared to t h e Zambezi, t h e y sustain a l a r g e p r o p o r t i o n
of t h e l a k e ' s primary production. Hence, n e a r l y 30000 t o n s of fish w e r e c a u g h t
between 1976 and 1981, of which some 80 p e r c e n t was t a k e n from t h e Sanyati basin
drained by one of t h e main secondary t r i b u t a r i e s of t h e K a r i b a l a k e (Kennmuir
1984). The evolution of t h e fish population i s similar to t h a t of most African manmade l a k e s showing a transition in time of r i v e r i n e t y p e of fish to lacustrinea d a p t e d cichlids, b u t also shows a s p e c i e s cline from o n e end of t h e l a k e (Mlibizi)
to t h e o t h e r (Sanyati). This confirms Rzoska's statement (Rzoska 1966) t h a t t h e
successive c h a n g e s in a newly c r e a t e d l a k e o c c u r o v e r a period of t h e o r d e r of t e n
to twenty y e a r s . Figure 33 summarizes t h e evolution of Lake Kariba in t e r m s of
ecological variables. The new l a k e underwent t h e e u t r o p h i c p h a s e a f t e r which i t
s h b i l i z e d i n t o t h e oligotrophic phase, and will p r o b a b l y r e t u r n to a shte of eutrophy u n d e r additional man-made influenaes in t h e f u t u r e .
SCALE 1 3 5 3 3 0 i o n o r o r I
-
Farrst
KEY
-
Arras
Communal Lana.
Oamr
ZAMBIA
ZIMBABHE
1 O l b m ~ n m t
I w
p
m
-
0 w t n Ioatm of fahlnt
ump m o w hDrr kdcatlon
of W l l l l . d q c w
4 O n e pm rcscne
... Rob.bk
I hundrm pm r t v n c
r sp-a -I
7
nune
Lh.Fn Plan area
h ~ oft urue fly
g m b n l o n p r to endcarton
ompr'P
1 L w m o n 01 t w m l la(nI8t~r~
Figure 34. A . T h e Sebungwe r e g i o n a n d land-use (Taylor, 198%); B. Detailed map
of vicinity d i s t r i c t s of L a k e K a r i b a a n d land-use (Magadza, 1986).
on
2.10. Other U r n of the h k e W
b
m Environs
The surroundings of t h e Zimbabwean p a r t of Lake Kariba belong t o t h e
Sebungwe region, a n area of about 40 000 km2 constituting 10 p e r c e n t of Zimbabwe
(Figure 34). Approximately 15000 km2 in t h e north of t h e region i s infested with
t h e tsetse fly. Low rainfall, unsuitable soils and marked slopes make this region a
marginal agricultural land (Taylor 1982). The fly f r o n t is demarcated by t h e
n o r t h e r n Sebungwe game fence. The region is divided between game r e s e r v e s ,
agricultural, and p a s t u r e areas (Figure 34). Although this region i s still sparsely
populated, man's influence o v e r t h e p a s t hundred y e a r s h a s been considerable.
The spectacular r u r a l population change in t h e sixties w a s t h e r e s u l t of t h e resettlement of t h e Tonga population, who initially lived in t h e Gwende valley, p r i o r to
t h e filling of t h e Kariba in t h e southern Sebungwe (Figure 35). Rural development
and wildlife conservation entail cohabitation problems due to t h e i r different
requirements. This conflict i s sharpened by t h e f a c t t h a t Zimbabwe e a r n s conside r a b l e foreign c u r r e n c y f r o m t h e tourist industry. Lake Kariba i s one of t h e
major a t t r a c t i o n s and nowadays t h e r e are l a r g e investments in hotel accomodation
and o t h e r r e l a t e d tourist facilities (Magadza 1986). The p r e s e n t population of t h e
Kariba town i s 13000. The majority of t h e employed a r e engaged in natural
r e s o u r c e s and wildlife r e l a t e d activities, such as fisheries, hunting, tourism, and
crocodile farming.
POPULATION INCREASE
National land
No increase
0-20%
40-60%
60-80%
1900
I
I
I
I
1920
1940
1960
1980
Year
Figure 35.Rural population change in the Sebungwe region (Taylor, 1982).
3. Cahora Bassa Impoundment
3.1. Historical Aspects
The idea of constructing a dam at Cahora Bassa w a s suggested a hundred y e a r s
ago, although f i r s t d i r e c t measures were t a k e n to implement t h e idea not e a r l i e r
t h a n 1956. The s i t e of t h e dam, t h r e e hundred kilometres from t h e mouth of t h e
r i v e r i s called "Kebra-Bassa", meaning "where t h e work stops" by slaves who came
o v e r 450 y e w s a g o with t h e Portuguese expeditions, where they were confronted
with falls a n d r a p i d s t h a t stopped them in t h e i r upstream progression (Gaster
1974). Construction of t h e second dam in t h e middle Zambezi commenced in t h e
C a h o m Bassa Gorge in 1969. The resulting l a k e began to fill on December 5, 1974.
The impoundment of Cahora Bassa l a k e began six months b e f o r e t h e independence
of Mozambique in 1975. Political arguments developed in P o r t u g u e s e publications
in o r d e r to point o u t t h e benefits t h a t t h e region would r e c e i v e from t h e l a r g e
impoundment, mainly as a driving f o r c e behind economic development taking into
account t h e r i c h n e s s of t h e tete region in coal, iron, chromium, nickel, manganese,
c o p p e r and aluminium (Hall and Davies 1974; Davies et erl. 1975). A t independence,
t h e v a s t sums of foreign c a p i t a l invested in t h e p r o j e c t were safeguarded by a P o r tuguese administrative company with t h e authority to o p e r a t e t h e dam, s e l l t h e
power and pay outstanding financial obligations (Bolton 1986). Thus, t h e sharing of
t h e Cahora Bassa dam between Portugal and Mozambique is 82 and 18 p e r c e n t ,
respectively. On August 2 , 1984, a n agreement w a s signed by Portugal, Mozambique
and South Africa stipulating t h a t two-thirds of t h e hydroelectricity produced at
Cahora Bassa should b e sold t o South Africa. In f a c t , since October 1983, almost
no e n e r g y h a s been produced d u e t o sabbotage of t h e o v e r h e a d transmission lines
leading from t h e dam to t h e only major consumer of t h e Republic of South Africa.
3.2. Main Featurea of the Man-Made Lake of Cahora Bassa
The t w o main r i v e r s flowing into t h e Cahom-Bassa r e s e r v o i r at Zumbo are t h e
Zambezi and t h e Huangwa. The catchment area of t h e Zambezi r i v e r a b o v e Cahora
Bassa l a k e consists of l o w hydrological subcatchments: (a) t h e Kariba catchment
( s e e C h a p t e r 3, Section 2). from which t h e releases from Kariba dam r a n g e
between 26 and 97 km3 p e r y e a r , with a n a v e r a g e of 51.5 km3; t h i s r e p r e s e n t s
almost 7 5 p e r c e n t of t h e total inflow into t h e C a h o m Bassa r e s e r v o i r ; (b) Kafue
catchment ( s e e C h a p t e r 3, Section 4) with a mean annual d i s c h a r g e of 10.3 km3
( m g e 2-30 km3); (c) Chongwe and Recomenche catchments along t h e north shore;
and (d) Chewore catchment along t h e south s h o r e of t h e Zambezi r i v e r between t h e
Kafue confluence and t h e Zumbo basin. Each of these catchments has an a r e a of
15000 km2. The o t h e r main r i v e r reaching t h e Cahora Bassa lake in Zumbo basin is
t h e Huangwa r i v e r . It has a drainage basin of 147 500 km2 in Zambia. The average
annual discharge is about 13 km3, but as t h e r i v e r is unregulated, floods peak
rapidly. About 90 p e r cent of t h e flow into Cahora Bassa r e s e r v o i r is regulated
(Kariba, Kafue river). Regarding t h e l a t e r a l inflows along t h e r e s e r v o i r s h o r e s ,
t h e total north s h o r e catchment is 16000 km2, while t h e south s h o r e i s 40 966 km2
(Bernacsek and Lopes 1984).
The Cahora Bassa r e s e r v o i r w a s formed with a 176-metre high wall and a n
area of about 2700 km2, with a n average depth of 26 metres. The lake consists of
seven basins (Figure 36). Its length and width are approximately 250 km and 38
km, respectively.
Table 30 and Annexes I and I1 give t h e main features of t h e
Cahora Bassa r e s e r v o i r .
7MLe YO. Main c h a r a c t e r i s t i c s of t h e Cabora k s s a r e s e r v o i r (Jackson and Davies
1976).
Cabora Bassa
Feature
Catchment
Geographical position:
longitude
latitude
Direction of main axis
Direction of predominant wind
Number of basins
Height of wall
Maximum depth
Mean depth
Maximum drawdown
Maximum length
G r e a t e s t width
Total area at capacity
Maximum floodgate discharge
Power capacity p e r turbine
Total power output of dam
Actual filling time
Impounded water m a s s
Infestant aquatic macrophytes
present in t h e system
m
m
m
m
km
km
km2
m 3sec- 1
MW
MW
years
m3
30°25'E-3Z044'E
15°29'S-26000'S
W-E
SE-NW
5
176
151
26
36
250
38
2 739
13600
430
3 870
--
1.S. molesta
2. P. s t r a t t o t e s
3. AaoLla n t l o t t c a
4. E t c h h o r n t a c r a s s t p e s
Figure 36.Cahora Bassa man-made lake (Bernacesk and Lopes, 1984).
of 7.1010n3 is intended f o r t h e production of 2 150
MW from five t r i b u t a r i e s . Another f o u r are planned f o r f u t u r e installation bringThe impounded w a t e r
ing a t o t a l p o t e n t i d power production of C a h o m
Bassa to 3 870 Mw
(Chapter 3,Sec-
tion 3.4).
3.3. Hydrology
The C a h o m Bassa l a k e l i e s e n t i r e l y in Mozambique, b u t t h e bulk of t h e
1000000 km2 d r a i n a g e basin l i e s in o t h e r countries. mainly Angola, Zambia and
Zimbabwe. The r e s e r v o i r consists of s e v e n basins from upstream to downstream:
t h e y are Zumbo, Messenguezi, Carinde, Mucanha, Mague, Chicoa and Garganta (Figu r e 36). S u r f a c e s of t h e s e basins as well as t h e catchment areas t h e y d r a i n are
indicated in Table 31.
Table 32. C a h o r a Bassa r e s e r v o i r basin s u r f a c e areas with waterlevel at 326000
a.s.1. a n d affluent catchment areas (Bernacsek and Lopes 1984).
North S h o r e
catchment
(km2>
Basin
surface
(km2>
South S h o r e
catchment
(km2)
3 598
Zumbo
330.5
25 215
639
86
Messenguezi
469
Carinde
82.6
9760
275
2 536
Mucanha
335.8
Mague
839.5
3 654
2 132
Chicoa
546.3
1657
333
Garganta
61.3
241
16 006
Total
40966
6 682
164
None of the 20 r i v e r s draining the total north s h o r e catchment into t h e reserv o i r i s perennial.
The t o t a l south s h o r e catchment i s l a r g e r t h a n t h e n o r t h .
Thirty-six r i v e r s e n t e r t h e l a k e including t w o perennials constituting 71.9 p e r c e n t
of t h e south s h o r e d r a i n a g e basin. These are t h e Hunyani, with a d r a i n a g e basin of
23897 km 2 e n t e r i n g t h e Zumbo basin and Messenguezi with a catchment area of
5556 km2, which e n t e r s t h e l a k e in t h e Messenguezi basin. Table 32 gives t h e rnorphometry of t h e C a h o m Bassa basins.
Trrble 32. Morphometry of the Cahora Bassa basins (Bernacsek a n d Lopes 1984).
Basins
Zumbo
Messenguezi
Carinde
Mucanha
Mague
Chicoa
Garganta
Extreme
length km
Mean
b r e a d t h krn
Surface
km2
Catchment
basin area
57.5
38.4
17.6
39.5
71.5
55.7
24.5
5.75
12.21
4.69
8.50
11.74
9.81
2.50
330.5
469.0
82.6
335.8
839.5
546.3
61.3
87.18
22.17
4.37
20.39
7.37
6.94
9.36
Concerning inflows into t h e C a h o m Bassa lake, t h r e e main p a r a m e t e r s must b e
t a k e n into account:
inflow of t h e t w o main t r i b u t a r i e s (Zambezi a n d Huangwa
r i v e r s ) , inflow of l a t e r a l r i v e r s , a n d precipitation o v e r t h e r e s e r v o i r area. Rainfall in t h e vicinity of t h e r e s e r v o i r area h a s been calculated as 650 mm p e r y e a r
a n d almost all rainfall o c c u r s between November a n d March. Evaporation f r o m t h e
l a k e s u r f a c e h a s a bimodal distribution peaking in April, a n d again m o r e strongly
in October. I t h a s been estimated at 4.73 km3/year (Bernacsek a n d Lopes 1984).
Inflows f o r l a t e r a l affluents are almost unknown but, taking into a c c o u n t t h e small
t o t a l catchment area (only 6.7 p e r c e n t of t h e total Cahora Bassa d r a i n a g e basin)
t h e i r contribution i s minor. Assuming a runoff coefficient of 0.20,a n d t h e annual
rainfall o v e r t h e region, t h e t o t a l l a t e r a l inflow volume affluents i s estimated at
7.41 km3/year. Inflow at Zumbo basin is t h e sum of flows of t h e t w o main inflow
s t r e a m s , t h e Zambezi a n d Huangwa r i v e r s (Figure 37). Due to t h e f a c t t h a t inflow
in Cahora Bassa i s 90 p e r c e n t r e g u l a t e d (Kariba, Kafue), t h e flood p a t t e r n h a s
changed s i n c e t h e s e impoundments.
he pro-Karlba
r i v e r showed a r e g u l a r annual
cycle, usually peaking in F e b r u a r y or March (5000-20000 rn3/s) a n d falling in
October-November to 200-800 rn3/s. The Kariba dam h a s r e s u l t e d in a n i n c r e a s e
in d r y season flows and a delay in t h e timing of floods during t h e w e t season. Thus,
t h e flood magnitude i s d e c r e a s e d by a n a v e r a g e of 24 p e r c e n t during t h e 1970-80
period, while b e f o r e the Kariba dam was built, t h e a v e r a g e r a t i o between t h e max-
imum a n d minimum flow of t h e Zambezi at Cahora Bassa Gorge was roughly 40:l.
Figure 37. Total monthly inflow in Zumba basin of Cahora Bassa l a k e (Bernacsek
a n d Lopes, 19B4).
3.4. Rememoir Operation
The main p u r p o s e of t h e C a h o r a Bassa r e s e r v o i r i s e n e r g y power generation;
dam o p e r a t i o n s e e k s to maximize e l e c t r i c i t y generation. The dam possesses eight
r e c t a n g u l a r flood g a t e s and five hydraulic t u r b i n e - a l t e r n a t o r s (Annex 11). The
t o t a l d i s c h a r g e capacity of t h e Cahora Bassa i s a b o u t 1 6 2 5 0 m3/s. Considering t h e
morphology of t h e Cahora Bassa r e s e r v o i r and i t s inflows a n d outflows, t h e l a k e
h a s a v e r y high o u t p u t p e r unit r e s e r v o i r area (1.4 h!w/km2) compared to t h e
K a r i b a (0.3 Idw/km2). The s t o r a g e r a t i o , which is t h e total r e s e r v o i r c a p a c i t y
divided by t h e mean annual inflow is 0.86 f o r t h e Cahora Bassa while i t i s 3.5 f o r
Kariba.
R o m t h e point of view of e l e c t r i c i t y generation, dam o p e r a t i n g pro-
c e d u r e s s e e k to maintain a r a t h e r constant water level of t h e l a k e , a n d depend on
t h e c h a r a c t e r i s t i c s of t h e dam (Figure 38): t h e water level cannot e x c e e d 330.5
m.a.s.1. a n d cannot d r o p below 295 m.a.s.1.
This gives a maximum working w a t e r
level r a n g e of 35.5 metres. Nevertheless, t h e water level should remain as close
to the maximum permissible level as possible in o r d e r to maximize t h e h e a d to t h e
turbines.
However, b e c a u s e of t h e r e s e r v o i r c a p a c i t y there i s n o a l t e r n a t i v e
I b C 53
- - -700 00-- 20700
1 :1 -
&mrnX/set OISCH.
17.00
RY S L A S O U R I V L I LCVCL A1 I.MO m Iw D l S C H
-10.00
------
D l V SCASDU l l V U LCVCL A 1
------
-!
1
Figure 38. Morphometric c h a r a c t e r i s t i c s of Cahora Bassa dam (Bernacsek and
Lopes, 1984).
o t h e r than to release water from t h e r e s e r v o i r before each rainy season t o
accomodate. and temporarily s t o r e , incoming flood waters which are then released
during t h e following d r y season. This entails important drawdowns as will be shown
below (Chapter 111, Section 3.5).
Since t h e constrbction of t h e Cahora Bassa dam in 1974. only two y e a r s (1980
and Lo a lesser e x t e n t 1979) c a n b e considered to have been "normal" hydrologically and operationally.
A combination of natural (hydrological) and socio-
political events have led to e r r a t i c behaviour. Furthermore, almost no electricity
has been produced at t h e Cahora Bassa power plant since October 1983 due to sabbotage of t h e transmission lines. The effect on t h e flow of t h e Lower Zambezi down-
stream from t h e Cahora Bassa dam is important (Figure 39).
Figure 39. Monthly maximum and minimum of t h e Zambezi at Tete below Cahora Bassa impoundment (Bolton 1986).
After 1974,when t h e Impoundment w a s completed, t h e r i v e r levels became f a r
more e r r a t i c and unseasonal. This can b e a t t r i b u t e d to t h e poor hydrological
operation of Cahora Bassa dam. Even If such problems may b e of a "relatively
s h o r t term" nature, t h e i r effects f r o m t h e ecological point of view are highly disruptlve. As a n example of "normal management", t h e p a t t e r n of annual fluctuation
in water level, water gain and loss are illustrated in Figure 40. Mainstream inflows
peak in March. Then floodgate discharge o c c u r s from May to July, but t h e peak is
in December-January.
Turbine discharges should b e virtually constant throughout
t h e y e a r and are roughly comparable to floodgate releases. Consequently, t h e
r e s e r v o i r level peaks in June and then falls to a mlnlmum dmwdown in January.
Water level, water gain and lose data f o r the period 1975-1982 are given in Annex
11, Sectlons 3.1-3.4.
. ll m
. I000
9W
a 00
.
100
.
hW
?
C
m
. 'w
YU
FER
MAR
LPR
M Y
UN
lliL
WG
SEP
OCT
WOV
DEC
5
.
LOO
.
1.00
-
-
0 00
u0)lrn
?m
40. Mean monthly water level, water gains a n d water losses from Cahora
Bassa r e s e r v o i r , a v e r a g e s f o r 1979 and 1980 (Bernacsek a n d Lopes, 1984).
3.5. Water Level Fluctuations
R e s e r v o i r s with a high output p e r unit r e s e r v o i r area like t h e Cahora Bassa
maximize power output f o r a given level of environmental disruption, but with
important consequences on r e s e r v o i r s t o r a g e r a t i o and drawdown. Thus, t h e maximum drawdown in normal o p e r a t i o n i s 34 m e t r e s which entails a l e s s s t a b l e aquatic
environment and a l s o poses more difficulties f o r shoreline habitation, f i s h e r i e s
a n d navigation (Bolton 1986). F o r water level at t h e maximum permissible level of
330.50 m e t r e s a b o v e sea-level, t h e Cahora Bassa l a k e h a s a n area of 3014 km2.
For t h e minimum operational water level, which is 295.0 m.a.s.1, t h e l a k e h a s a n
area of 838 km2. Thus, a c t u a l drawdown of 34 m e t r e s entails tremendous variations
in t h e area of Lake water s u r f a c e . Figure 41 gives t h e relationship between l a k e
waterlevel and surface area and t h e proof. These c u r v e s h a v e been plotted from
d a t a given in Annex 11, Section 4. Due to t h e f a c t t h a t t h e predictability of flood
timing and size i s p o o r , drawdowns must b e of sufficiently high magnitude to
MEAN DEPTH ( m l
5
0
10
D A M CREST
25
20
15
30
35
LO
~krn'r
50
55
L5
60
70
65
331.00m a.s. 1.3
fl
I
'
.
l
.
-
1,000
5 00
.
A
1
.
'
1,500
a
.
l
.
'
"
2,000
l
A
a
.
2,500
.
.
.
3, WO
SURFACE AREA ( krny I
Fi.an 41. Cahora Bassa r e s e r v o i r s u r f a c e a r e a a t water levels 295.00 to 331.000
m.a.s.1. (Bernacsek and Lopes, 1984).
incorporate a significant safety margin. They must allow sufficient s t o r a g e capacity for incoming floods as well as r e t a i n sufficient water in o r d e r to maximize
hydropower production.
The theoretical design for water level fluctuations for
t h e Cahora Bassa i s given in Table 33.
W L e 33. Maximum permissible water level at t h e end of each month on t h e Cahora
Bassa r e s e r v o i r (Bernacsek and Lopes 1984).
Month
Maximum
Month
Maximum
water level
m.a.s.1.
July
August
September
October
November
December
324.7
323.4
322
320.6
319.1
317.6
water level
m.a.s.1.
January
February
March
April
May
June
316
321
326
326
326
326
Theoretically, t h e water level should fluctuate around a median of 321 m.a.s.1.
within a ten-metre r a n g e , which is significantly higher than t h e median of 312.75
m.a.s.1. between dead s t o r a g e (295 m.a.s.1.) and maximum permissible level (330.5
m.a.s.1.).
A 10-metre fluctuation i s excessive from a f i s h e r y p e r s p e c t i v e (Chapter
111, Section 3.9). I t i s clear t h a t flood prediction must include close and continual
liaison with t h e power plant managers and hydrologists of t h e Kariba, Kafue and
Itezhi-Tezhi dams, permitting t h e reshaping of t h e design c u r v e . It a p p e a r s t h a t it
would b e possible t o r e d u c e t h e magnitude of annual fluctuations t o 3 o r 4 metres
and r a i s e t h e median level to 324 m.a.s.1. (Bernacsek and Lopes 1984). Variations
in r e s e r v o i r water level from 1975-1982 are plotted in Figure 42. The observed
changes in water level are f a r from t h e design operation c u r v e and show t h e seasonal floods as well as d r a s t i c drawdowns.
1::
x
128
T
.
'>tx
1 7 . OC
1 1 7 x
11c
m
OC
I I b OC
18' 0 0
112 OC
11C OC
1CtOS
106 ffi
i c , a?
rczm
l'x DO
rrtm
29630
Figure 42. Variations of water level in Cahora Bassa r e s e r v o i r (Bernacsek and
Lopes, 1984).
3.6. Water Chemistry
Physim-chemical assessments of t h e Zambezi r i v e r p r i o r to t h e c l o s u r e of t h e
Cahora Bassa dam (Hall 8t d. 1977) showed t h a t t h e water quality of t h e Zambezi
r i v e r entering Mozambique was mainly determined by outflows of t h e Kariba and
t h e Kafue dams. Since t h e c l o s u r e of t h e dam, i t h a s been noticed t h a t t h e r e i s a
substantial i n c r e a s e in dissolved substances between Kariba dam and t h e Zumbo
basin f o r during t h e passage of water through t h e Cahora Bassa r e s e r v o i r , t h e r e
i s a n i n c r e a s e by a f a c t o r of 1.24 f o r t o t a l dissolved solids (97.47 t o 120.67 mg/l)
and of 1.03 f o r conductivity (114 to 117 j6). This i n c r e a s e in conductivity i s small
%ble 34. Main physico-chemical c h a r a c t e r i s t i c s of t h e Cahora Bassa lake (Bernacsek and Lopes 1984).
Parameter
Unit
Mean
Ranee
Cations
Na
Ca
Anions
C1
HCO,
SiO,
Organf c m a t t e r
Dissolved solids
Dissolved gases
mg/l
mg/l
pH units
Turbidity
Species
Jackson
turbidity
unit
Mean concentration
(mg/l)
Milliequivalent
Cations:
Calcium
Sodium
Magnesium
16.08
6.08
4.16
0.401
0.264
0.171
Totals
26.32
0.836
Anions:
Bicarbonate
Chloride
Sulphate
70.54
8.66
6.69
0.578
0.244
0.070
Totals
85.89
0.892
oompnred to Ule value obtained in Kariba (1.36) (Bernacsek and Lopes 1984).
Table 34 gives t h e main physim-chemical c h a r a c t e r i s t i c s of t h e Cahora Bassa
reservoir.
W b l e 35. Average values (and s t a n d a r d deviation) of middle a n d lower Zambezi
(Hal1 #t d . 1977).
Parameters
Units
Huangwa
Water temperature
pH
i is solved oxygen
Transparenay
Total alkalinity
Chloride
Calcium hardness
Total hardness
Colour
Conductivity
Ammonia
Nitrites
Nitrates
Orthophosphates
Silica
Sulphates
Suspended solids
Na
K
Ca
Mg
Fe
Mn
Stations
Shire
Hunyani
Messenguezi
mg 1;' ~ 0 ,
mg 1- SiO,
mg I-' SO,
On t h e o t h e r hand, t h e influence of t h e main t r i b u t a r i e s o n t h e w a t e r quality
of t h e Cahora Bassa r e s e r v o i r i s q u i t e important, mainly in t h e vicinity of t h e i r
confluence. Their importance i s enhanced d u e to t h e high concentration of dissolved matter (Table 35).
The influence of t h e S h i r e o n t h e w a t e r quality of t h e Zambezi r i v e r downs t r e a m Cahora Bassa dam h a s b e e n discussed by Hall et al. (1977). Due to t h e s h o r t
r e s i d e n c e time of w a t e r in t h e C a h o r a Bassa r e s e r v o i r (0.61 y e a r ) , t h e e f f e c t of
l a t e n t h e a t of impounded w a t e r mass in t h e r e s e r v o i r d o e s not e x e r t as g r e a t a n
e f f e c t as in t h e Kariba r e s e r v o i r , which h a s a longer r e s i d e n c e time (4.0 y e a r s ) .
Thus, Bernacsek a n d Lopes (1984) consider t h r e e p h a s e s of thermal evolution of
impounded w a t e r in t h e C a h o r a Bassa r e s e r v o i r : (a) filling p h a s e
- t h e peak
inflow
at Zumbo basin t a k e s p l a c e in March-April during which time almost 24.26 km3
( r e p r e s e n t i n g 43.5% of t h e t o t a l r e s e r v o i r volume) e n t e r t h e r e s e r v o i r .
This
incoming m a s s of relatively warm w a t e r (25°C from t h e Kariba metaliminion) would
r e ~ h c et h e r e s i d e n t water mass of the f o u r western basins (from Zumbo to
Mucanha). High wind velocittes mainly from the s o u t h e a s t in March tend to k e e p
t h e new w a t e r mass well mixed, Which makes s t r a t i f i c a t i o n in t h e s e shallow basins
difficult; (b) cooling p h a s e
- by t h e middle of
t h e y e a r , the a i r t e m p e r a t u r e and
water inflow r ~ c minima.
h
I n c r e a s e d d i s c h a r g e through Cahora Bassa dam from
May to July (22.15 km3 or 39.7% of t h e t o t a l r e s e r v o i r volume at 326.00 m.a.s.1.)
would draw t h e cooled and well mixed mass of water i n t o t h e e a s t e r n basins; (c)
heating p h a s e
- a i r temperatures reach
t h e i r maximum in t h e l a t e r months of t h e
y e a r resulting in heating of t h e s u r f a c e l a y e r of t h e r e s e r v o i r . Wind may p r e v e n t
t h e development of s e v e r e stratification, and tend to i n c r e a s e t h e d e p t h of t h e
heated u p p e r l a y e r t h r o u g h mixing. In t h e d e e p e r e a s t e r n basins, s t r a t i f i c a t i o n
should b e b e t t e r developed.
Another c h a r a c t e r i s t i c of t h e Cahora Bassa r e s e r v o i r i s t h e intense olive
g r e e n c o l o u r of t h e water (Gliwicz 1982) t h a t p e r s i s t s t h r o u g h t h e y e a r reducing
light p e n e t r a t i o n dramatically and preventing growth of phytoplankton. Regarding
t h e dissolved gases, Bond et arl. (1978) d e t e c t e d oxyclines in t h e latter p a r t of t h e
y e a r in e a s t e r n basins which c o r r e s p o n d s to t h e thermal s t r a t i f i c a t i o n . Meanwhile,
the western basins d o not develop oxyclines d u e to the well oxygenated Zambezi
inflow and wind-induced mixing. In order to deduce water quality in t h e Cahora
Bassa r e s e r v o i r , it a p p e a r s t h a t the s h o r t r e s i d e n c e time of t h e water e n t a i l s a
weaker influence of t h e impoundment on t h e physico-chemical evolution t h a n in t h e
case of t h e Kariba, f o r instance.
3.7. Sediment Deposition
N o quantitative estimate of t h e rate of sediment accumulation in t h e C a h o r a
Bassa h a s been published. However, some estimates h a v e been published including
o n e r e p o r t e d by Bolton (1984). Regarding t h e input of sediment to t h e r e s e r v o i r , a
l a r g e p a r t of t h e catchment area of t h e Cahora Bassa r e s e r v o i r c a n b e neglected:
t h e K a r i b a dam r e g u l a t e s almost 65%of t h e d r a i n a g e basin t r a p p i n g a l l i t s incoming
sediment, while t h e Kafue dam r e g u l a t e s a f u r t h e r 15 p e r c e n t of t h e e n t i r e basin
of the C a h o r a Bassa lake. T h r e e d i f f e r e n t subcatchments should b e considered t o
have a n impact o n t h e sediment input in t h e lake: (a) t h e basins of t r i b u t a r i e s in
Zimbabwe (42.10'km2),
(b) t h e basins of minor t r i b u t a r i e s in Zambia a n d Mozam-
bique (35.103km2), a n d ( c ) t h e Huangwa basin (148.103km2). The Huangwa basin in
Zambia i s t h e principal source of sediment to t h e C a h o r a Bassa. Erosion rates
h a v e been a c c e l e r a t e d by c h a n g e s in land u s e s i n c e t h e t u r n of t h e c e n t u r y . It i s
r e p o r t e d t h a t sediment concentration in t h e r i v e r r a r e l y falls below 1 0 0 0 mg/l.
Thus, i t i s believed t h a t sediment from t h e Huangwa r i v e r to t h e C a h o r a Bassa
r a n g e s between 15 and 1 5 0 . 1 0 ~t / y e a r . Concerning t h e basins of minor t r i b u t a r i e s
in Zambia a n d Mozambique, Bolton (1984) suggests t h a t sediment is of t h e o r d e r of
4
-xr+
rbG Ctaboro Basso
.
Karlba
\
l k o ~ n ae b a s ~ nof
Lake aboro Bassa
In
Zimbabwe
~ c n r ~ cruslon
a l
hazard
based an Stocking ond Elwell
L'my
low lo lav
- --
-
-
Ele~l~an
(rn oms1 I
-.
- --- - - - - - - - - r
Normal malrnum
- - - -owollng
- -- -level
- - L-326
- m- oms
- - l-
kcanpadze Barn
Lhstame lmm dom (bnI
Figure 43. Potential erosion hazard in W o n Bassa tributary basins in Zimbabwe
and longitudinal profile of C a h o m Bassa r e s e r v o i r (Bolton, 1984).
200 t/km2, and sediment input rate f r o m these t r i b u t a r i e s into t h e Cahora Bassa
should r a n g e between 3 and 3 0 . 1 0 ~t/year. Finally, taking into account t h e r e s u l t s
of Stocking and Elwell (1973), i t a p p e a r s t h a t t h e Cahora Bassa's t r i b u t a r i e s in
Zimbabwe (Figure 43) should provide total sediment discharge which ranges
between 2 and 2 0 . 1 0 ~t/year.
Making t h e same assumptions f o r t h e consolidation of deposits as for t h e
Kariba, t h e loss of s t o r a g e capacity lies in t h e range of 20 and 2 0 0 . 1 0 ~t / y e a r .
The sediment input rate to t h e Cahora Bassa r e s e r v o i r a p p e a r s to b e a f a c t o r
t h r e e times g r e a t e r than t h a t of Lake Kariba. I t s s t o r a g e capacity i s considerably
smaller. Considering t h e design of t h e Cahora Bassa p r o j e c t a minimum drawdown
level of t h e r e s e r v o i r i s taken to b e 295 m.a.s.l., and t h e "dead" s t o r a g e below this
level i s almost 12.5.10 9 m 3. This would b e filled in a period of 60-600 y e a r s if all
sediment would b e deposited in t h e dead s t o r a g e , although due to t h e p a r t i c u l a r
s h a p e of t h e Cahora Bassa Lake (Figure 43). a significant proportion of t h e incoming sediment will accumulate within t h e "line" s t o r a g e of t h e r e s e r v o i r . The bulk of
t h e incoming sediment passes through a broad shallow basin (Mucangadze basin)
which is separated from t h e d e e p e r parts by a n a r r o w r e a c h constricted by
islands. Thus, sediment deposit in the Cahora Bassa r e s e r v o i r should affect i t s
operation much sooner than expected.
3.8. Ecological Aspects
The Cahora Bassa r e s e r v o i r has, as mentioned above, a high output p e r unit
r e s e r v o i r area for i t i s a kind of a "riverine" r e s e r v o i r as compared t o a relatively s t a b l e 'lacustrine" one such as t h e Kariba. The r i v e r i n e influence i s more
accentuated within t h e lake, especially in t h e western basins. Furthermore, its
s h o r t water m a s s t u r n o v e r cannot lead to lacustrine conditions e x c e p t in t h e
e a s t e r n basins. During t h e filling phase of t h e Cahora Bassa r e s e r v o i r between
December 1974 and March 1975, downstream flow of t h e Zambezi r i v e r h a s been
reduced from a n a v e r a g e discharge of 3000 to 60 m3/sec (Davies 1975a.b). This
flow s h o r t a g e certainly h a s had ecological consequences t h a t persisted beyond t h e
only filling period. On t h e o t h e r hand, t h e duration of t h e initial "productive
phase" h a s been s h o r t e r than t h e Kariba, certainly due to a more r a p i d lake water
exchange (Bond e t crl. 1978). So no special massive release of minerals from
drowned t e r r e s t r i a l habitat have taken place during t h e f i r s t y e a r of t h e life of
t h e Cahora Bassa. The brief duration of maximum flooding probably did not allow
sufficient time f o r decomposition of plant matter to o c c u r . Thus, expected aquatic
macrophyte infestation (Davies e t crl. 1975) as seen in t h e case of t h e Kariba did
not o c c u r . Only 0.32 p e r c e n t of t h e e n t i r e r e s e r v o i r s u r f a c e w a s covered with
water hyacinth ( E t c h h o m i a c r a s s i p e s ) .
I t seems t h a t t h e small r e l e a s e of
minerals from flooded t e r r e s t r i a l environments, wind-wave action as well as imporh n t drawdown have hindered a l a r g e infestation of t h e lake s u r f a c e with aquatic
m c r o p h y t e s (Bond a n d R o b e r t s 1978). A marked hydrological heterogeneity along
t h e east-west and t h e north-south a x e s of t h e Cahora Bassa l a k e i s a p p a r e n t . The
east-west heterogeneity c o n c e r n s transition from r i v e r i n e t o lacustrine environmental conditions within subbasins, while t h e north-south heterogeneity i s d u e to
t h e prevailing wind (NWSE and N-S, respectively, 35 and 20% of f r e q u e n c y ) causing d i f f e r e n c e s in t e m p e r a t u r e and oxygenation within t h e r e s e r v o i r .
3.9. Fish Population
A total of 58 fish s p e c i e s was recognized from t h e middle Zambezi b e f o r e t h e
c l o s u r e of t h e C a h o r a Bassa. Among them almost 4 3 p e r c e n t , o r , n e a r l y half of t h e
total number consisted of two s p e c i e s of t h e family Charicidae, t h e small carnivorous Alestes i m b e d and t h e piscivorous Hydrocynus v i t t a t i s (Jackson and
R o g e r s 1976). The c l o s u r e of Cahora Bassa dam in December coincided with t h e
seasonal spawning migration up inflowing r i v e r s , of many cyprinid, c h a r a c i d , silur o i d a n d o t h e r non-cichlid fish species, a n d consequently a v e r y high survival of
t h e newly spawned juveniles a c c u s e d in t h e s e newly flooded area w h e r e t h e y could
find b e t t e r p r o t e c t i o n from p r e d a t o r s and more abundant food t h a n in t h e old confined r i v e r (Jackson and Davies 1976). Nowadays 33 s p e c i e s of fish h a v e been
recognized in t h e C a h o r a Bassa r e s e r v o i r , r e p r e s e n t i n g many zoological levels (25
g e n e r a and 13 families) (Bernacsek a n d Lopes 1984). Among t h e s e families Charac i d a e are still well r e p r e s e n t e d with Hydrocynus v i t t a t u s ( t i g e r fish) and Alestes
species. From t h e Dishtichodotridae family, Distthodus scheuga i s most common.
In t h e Chicoa basin f o r instance it i s t h e dominant consumer of primary production
a n d forms a major p r o p o r t i o n of t h e gillnet c a t c h . Among t h e Cyprinidae family,
f o u r s p e c i e s of Labeo e x i s t in t h e C a h o r a Bassa basin. They are a common component of commercial c a t c h , especially in t h e western basins. Concerning t h e Clupeidae family, L i m n o t h d s s a mtodon s p e c i e s introduced from Lake Tanganiika to
t h e Kariba is now well established in t h e Cahora Bassa r e s e r v o i r . I t i s assumed
t h a t t h e introduction w a s natural. P r i o r to t h e impoundment at Cahora Bassa t h e r e
w e r e speculations as t o w h e t h e r or not L i m n o t h d s s a was c a p a b l e of establishing
itself by travelling down t h e Zambezi r i v e r from t h e Kariba (210 km) (Kenmuir
1975; Jackson a n d Davies 1976). Thus Ltmnothrtssa is now well established at
least in t h e e a s t e r n l a c u s t r i n e basins of t h e Cahora Bassa (Bernacsek and Lopes
1984). The b a s i c i n f r a s t r u c t u r e of commercial f i s h e r i e s on t h e Cahora Bassa i s
v e r y limited in comparison to o t h e r major African r e s e r v o i r s . This i s d u e mainly
to t w o f a c t o r s : (a) an extended p e r i o d of w a r f a r e in t h e a r e a , and (b) o t h e r
government p r i o r i t i e s (marine fisheries). Nevertheless, a t o t a l yield of 4 3 4 3 tons
of t h e fish h a s been estimated f o r Cahora Bassa in 1982, while a t o t a l annual yield
of s a r d i n e s could b e approximately 6 0 0 0 tons (Bernacsek a n d Lopes 1984). It i s
a l s o estimated t h a t t h e potential yield f o r Cahora Bassa could b e 6 700 tons of t a b l e
fish and 8 0 0 0 tons of s a r d i n e s p e r y e a r . These estimates seem to b e close to calculations based on t h e Morpho Edaphic Index (MEI) developed f o r o t h e r man-made
l a k e s in Africa (Figure 44). This index estimates a n annual yield from water
conductivity of t h e r e s e r v o i r a n d i t s mean d e p t h according to t h e following equation: yield
= 23.3956 M E I O . ' ~ ~ ~e x p r e s s e d in kg/ha/year; ME1 = conductivity in
p.5
divided by m e a n d e p t h (metres).
'OoO
1
t
B R E S I I O N EQUATIONS
RESERVOIRS
:
23.1956 l H E l o'A"'l
f
1
I0
100
-
1
toot
UORPHO EDAPHIC INDEX
Figure 44. Relationship between morpho-edaphic index (MEI) a n d annual fish yield
f o r African r e s e r v o i r s (Bernacsek a n d Lopes, 1984).
The conductivity of C a h o r a Bassa is 117 p.5 and t h e mean d e p t h i s 20.92 m at
r e f e r e n c e level 326 m.a.s.1.
This gives a n ME1 of 5.59.
The yield calculated
according to t h i s formula i s 48.32 kg/ha/year equivalent to a n annual c a t c h of
1 2 8 7 7 tons. This is quite in a c c o r d a n c e with estimates from Bernacsek and Lopes
(1984). The ME1 Index i s a n interesting tool f o r generating estimates of r e s e r v o i r
yields a n d total c a t c h at d i f f e r e n t water levels. Figure 45 gives t h e relationship
between water level and t o t a l annual c a t c h in Cahora Bassa r e s e r v o i r . I t c a n b e
noticed t h a t within t h e r a n g e of 310.0-330.5 m.a.s.l., t h e relationship i s approximately linear. An i n c r e a s e in m e a n water level from, f o r instance, 322 to 325 m
would theoretically i n c r e a s e t h e c a t c h by 709 tons. Thus, i t i s clear t h a t from t h e
f i s h e r y p e r s p e c t i v e , t h e design water level fluctuations of o v e r 10 metres is unacceptable. The b e s t i n t e r e s t s of t h e f i s h e r i e s are s e r v e d by t h e highest possible
mean o p e r a t i n g level 324 m e t r e s a.s.1. with a small annual fluctuation (3 o r 4
metres).
/
/
---tam
eLAO
tm
-- ----_-____-LlWbGC LtVLL
----.-14m
1 0 . 0 0 m r &1
- I . - a _ . . . - . . - -
11,000
u.ooa
PREDlClED TOTAL CATCH l t o n s l y r 1
Y
-
.
.
.
I
-
-
-
I&WO
-
-
-
-
.
I
~
.
"
.
1L.000
Figure 45. Relationship between water level of C a h o m Bassa l a k e and total potential annual c a t c h p r e d i c t e d by ME1 equation f o r African r e s e r v o i r s (Bernacsek and
Lopes, 1984).
3-10.Other Purposes
The Cahora Bassa p r o j e c t ' s potential to transform t h e region remains high,
a n d o n c e t h e unique short-term political and economic situation improves, benefits
could b e substantial (Bolton 1984). As y e t , however, t h e far-reaching multiple purp o s e benefits (i .e. e n e r g y power, mining, irrigation, tourism) envisaged by t h e
p l a n n e r s h a v e not materialized. Even concerning t h e main p u r p o s e of t h e Cahora
Bassa impoundment, e l e c t r i c a l power generation, t h e o u t p u t is v e r y small. P r a c t i cally t h e only p r e s e n t u s e is t h e a r t i s a n a l fishery. Nevertheless, if t h e estimate of
fish yield i s high, t h e real h a r v e s t i s f a r from t h e existing potential.
4. Impoundments on the Kafue River
4.1. Historical Aspects
The Republic of Zambia i s o n e of t h e most industrialized countries of t h e Zambezi
drainage basin. Copper mining dominates t h e economy of Zambia and adequate supplies of low-cost electricity are vital t o t h e country, of which t h e c o p p e r industry
is a n extensive user. Copper mines are located mainly in t h e u p p e r p a r t of t h e
Kafue drainage basin. Until 1972, electricity produced in Zambia came from t h e
Victoria Falls power plant and from t h e Kariba dam from which hydropower output
w a s s h a r e d with Rhodesia (now Zimbabwe). Thus, Zambia's requirements of power
were m e t from s o u r c e s outside t h e country s h a r e d with o t h e r s . A s a consequence
of reducing i t s supplies caused by conflicting interests, Zambia s t a r t e d to plan f o r
self-sufficiency. The Kafue r i v e r i s in f a c t a v e r y a t t r a c t i v e system from t h e
energy power production point of view.
A s t h e r i v e r belongs entirely to t h e
Republic of Zambia, i t s drainage basin includes t h e c o p p e r belt area and t h e r e is a
rapid d r o p of t h e r i v e r in t h e gorge t o give a good impoundment zone, Zambia
decided t o construct a power plant on t h e Kafue Gorge. The f i r s t dam was c o m pleted on t h e Kafue Gorge below t h e Kafue swamps in 1972. L a t e r on, in o r d e r t o
g e t sufficient s t o r a g e without a g r e a t increase in s u r f a c e a r e a , a second dam w a s
built at Itezhi-Tezhi at t h e u p p e r end of t h e flats in 1977.
4.2. Main Features of the Kafue Impoundments
The Kafue r i v e r i s almost 1 5 0 0 km in length up to i t s confluence with t h e Zambezi r i v e r , with a catchment a r e a of about 155000 km2 up t o t h a t point (Chapter 1,
Section 2.3; Figure 46). The Kafue r i v e r i s one of t h e major t r i b u t a r i e s of t h e
Zambezi r i v e r . I t s drainage basin can b e subdivided into t h r e e subbasins: (a) t h e
u p p e r half of t h e catchment area in t h e wetter n o r t h e r n p a r t of Zambia, roughly
north of Itezhitezhi, (b) t h e middle, area r e f e r r e d to as t h e "Kafue flats" from
Itezhitezhi to t h e Kafue Gorge, and ( c ) t h e lower p a r t of t h e r i v e r , t h e Gorge
where t h e r i v e r d r o p s steeply from t h e plateau to t h e level of t h e Zambezi r i v e r
(Figure 46 and 47). Taking advantage of t h i s geomorphological f e a t u r e , t h e f i r s t
dam was built in t h e Kafue Gorge. Water from t h e r e s e r v o i r i s c a r r i e d through a
h e a d r a c e tunnel of 1 0 km, and then falls vertically f o r 410 metres to t h e power
plant just above t h e r i v e r (Sheppe 1985).
Figure 46. K a f u e drainage basin and K a f u e flats ( R e e s 1978b and Handlos 1982).
The catchment a r e a above t h e dam is about 150000 km2, providing a mean
annual inflow of 10.3.10 9 .m 3. This impoundment generates power with four turbines
with a maximum discharge capacity of 200 m3/sec (Annex I). A s m o s t of t h e reservoir i s in t h e Kafue flats, t h e s t o r a g e capacity i s low and t h e s u r f a c e a r e a is large,
resulting in g r e a t water loss through evapotranspiration. Then, in o r d e r to provide sufficient s t o r a g e without a g r e a t increase in s u r f a c e area and water losses, a
second dam w a s built at Itezhitezhi (Figure 46) in 1977. It forms a l a r g e r e s e r v o i r
with 8 . 5 . 1 0 ~rn3 capacity which provides m o s t of t h e s t o r a g e capacity f o r t h e Kafue
Gorge power plant (Annex I).
Figure 47. Longitudinal profile of t h e Kafue r i v e r (Handlos, 1982).
4.3. Hydrology
In view of t h e drainage basin of t h e Kafue r i v e r , t h e r e i s a g r e a t difference in
t h e runoff contribution between t h e u p p e r p a r t of t h e catchment (upstream Kafue
Hook) and t h e lower p a r t . The rainfall in t h e u p p e r p a r t s of t h e catchment i s m o r e
than in t h e lower p a r t ; hence t h e contribution to t h e runoff in t h e r i v e r i s m o r e
from t h e u p p e r regions. Thus, 93240 km2 covering t h e u p p e r p a r t of t h e catchment a r e a provide a n annual a v e r a g e runoff of about 10.109 m 3, while t h e drainage
basin lying between Itezhitezhi and t h e Kafue Gorge (59569 km2) provide only a n
a v e r a g e runoff of 1 . 1 . 1 0 ~ m ~ .Differences in precipitation amounts between t h e s e
t w o p a r t s of t h e Kafue drainage basin as well as t h e important evapotranspiration
which o c c u r s in t h e Kafue f l a t s increase t h e differences in runoff contribution by
them. Thus, i t i s r e p o r t e d t h a t in t h e upper drainage basin, t h e a v e r a g e rainfall i s
close to 1 4 0 0 mm/m2 annually, while in t h e flats, rainfall a v e r a g e s 800 mm/rn2.
Estimates of evapotranspiration in t h e Kafue f l a t s are about 2700 mm/m2 p e r
annum.
The balance between precipitation and evapotranspiration i s negative
within t h e Kafue flats.
Figure 48. Discharge a t Itezhitezhi (broken line) and Kasaka (solid line) from 1950
to 1965 (1952-53 missing). Each hydrological y e a r begins in October (Handlos,
1982).
On average, significant amounts of r a i n in t h e flats begin about mid-November
and continue until about t h e middle of March. Rainfall and runoff from t h e local
t r i b u t a r i e s account f o r t h e initial flooding and waterlogging of t h e Kafue f l a t s
between December and February (Handlos 1982). Rainfall from t h e n o r t h e r n
catchment runoff to Itezhitezhi and peak in March, and then flows slowly southwards, finally reaching a peak at Kafue Gorge in May, often well a f t e r t h e local
r a i n s in t h e f l a t s have ended. The r a i n s in t h e n o r t h e r n p a r t of t h e catchment con-
tinue until mid-April, t h u s continuing to f e e d water into t h e Kafue r i v e r f o r almost
a month a f t e r t h e local r a i n s in t h e Kafue f l a t s h a v e ended (Hutchinson 1974). A s a
r e s u l t , high water levels are maintained on t h e Kafue f l a t s f o r 6-7
months
(December to June). T h e r e f o r e , much of t h e flooding between March a n d June i s
due to t h e runoff from t h e n o r t h e r n p a r t of t h e Kafue d r a i n a g e basin. The Kafue
r i v e r meanders f o r a distance of 410 km as i t t r a v e r s e s t h e 250 km distance a c r o s s
t h e Kafue flats. The normal channel of t h e r i v e r i s estimated t o b e a b l e t o c a r r y
170 m3/s of water, but during t h e height of t h e flood (2500 m3/s b e f o r e t h e Itezhitezhi impoundment), t h e water spills into t h e Kafue f l a t s area a n d floods t h e f l a t s if
t h e y have not a l r e a d y been inundated by local rainfall a n d r u n o f f , or maintains t h e
a l r e a d y o c c u r r i n g flood. Figure 48 gives d i s c h a r g e s at Itezhitezhi and Kasaka
(Kafue Gorge) b e f o r e t h e Kafue impoundments.
4.4. Reservoir Operation
The main p u r p o s e of t h e s e two impoundments on t h e Kafue r i v e r i s power gene r a t i o n , even if planned r e s e r v o i r o p e r a t i o n t r y to t a k e into account some o t h e r
secondary goals (Howard a n d Williams 1982). Among t h e f a c t o r s favouring Itezhitezhi as t h e s i t e f o r t h e main backup s t o r a g e f o r t h e g o r g e power station, i s t h e
f a c t t h a t i t i s a b l e to c o n t r o l practically t h e e n t i r e yield of t h e r i v e r (almost 90
p e r c e n t of t h e flow). However, o n e of t h e major disadvantages of t h e Itezhitezhi
s i t e i s t h a t t h e t r a v e l time to t h e Kafue Gorge f o r water r e l e a s e d from Itezhitezhi
i s a b o u t two months, a factor t h a t a d d s to t h e complexity of r e s e r v o i r operation.
F r o m t h e point of view of power generation, production of 600 Mw in t h e g o r g e
power s t a t i o n needs a r e g u l a t e d flow corresponding to 170 m3/sec at t h e g o r g e
~ Itezhitezhi
(Balasubrahmanyam a n d Abou-Zeid 1982a). A s t o r a g e of 4 2 0 0 . 1 0 ~ min
r e s e r v o i r was decided as t h e b a s e amount f o r power production in t h e Kafue Gorge
power plant. To t h i s w a s added a s t o r a g e of 750.106m3 f o r flooding t h e f l a t s even
in t h e d r y y e a r s in r e s p o n s e to ecological dictates. Together with a d e a d s t o r a g e
of 750.106m3, t h e g r o s s s t o r a g e works o u t to b e 5 7 0 0 . 1 0 ~ m ~During
.
a "normal"
y e a r b e f o r e t h e Itezhitezhi dam w a s built, t h e flow in t h e western f l a t s r a n g e d from
30 to 1400m3/sec with r e c o r d e d e x t r e m e s of 1 0 a n d 2 7 0 0 m3/sec. F o r normal
o p e r a t i o n , i t w a s stipulated t h a t t h e Itezhitezhi r e s e r v o i r should r e a c h i t s full
r e t e n t i o n level of 1023.5 m in J a n u a r y or F e b r u a r y depending on t h e hydrological
conditions in t h e y e a r (Balasubrahmanyam and Abou-Zeid 1982b). Once t h e reserv o i r r e a c h e d i t s full r e t e n t i o n level, practically a l l t h e inflow into Itezhitezhi
should be released downstream (Figure 49). During d r i e r y e a r s t h e flow p a t t e r n
would b e as in Figure 49.
I ,
I..,.
OC I
'
hOv
'
DEC
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1.-
LAY' J U U
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.
.
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Figure 49. The Kafue hydrograph f o r a n a v e r a g e y e a r at Itezhitezhi (above) and
during a d r y y e a r (below) (Balasubrahmanyan and Abou-Zeid, 1982b).
Only regulated releases to m e e t power generation requirements, plus t h e 1 5
m3/sec stipulated f o r o t h e r uses are made monthly, e x c e p t in March when 300
m3/sec i s released to maintain at least partial flooding of t h e f l a t s (White 1973;
Balasubmhmanyam and Abou-Zeid 1982b). However, i t a p p e a r s t h a t in m o s t y e a r s ,
r e s e r v o i r operation follows t h e second p a t t e r n (Sheppe 1985). Nevertheless,
Itezhitezhi r e s e r v o i r o p e m t i o n should provide optimum water to t h e Kafue Gorge
power plant (Figure 50).
.
h f o m It~zh~lezh .
9"
'
9"
.
\
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Follorlng rules OI operoljln
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.
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and
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1173
1171
19 75
1976
'9:-
Figure 50. Water s u r f a c e elevation at t h e Kafue Gorge dam (Balasubrahmanyan
and Abou-Zeid, 1982b).
4.5. Water Level Fluctuations
The Kafue flats c a n b e divided into t w o main p a r t s , from Namwala t o Nyimba
and from Nyimba to t h e Kafue Gorge. The second (eastern) section is affected by
t h e fluctuation of t h e Kafue Gorge r e s e r v o i r .
Normal operation of t h e dam
envisages maintenance of t h e Kafue Gorge r e s e r v o i r a t t h e 975.3 m level from
August t o November, rising t o t h e full retention level of 976.6 m in December,
maintaining this level until t h e following March. Thus, inundation of areas in t h e
section Nyimba to Kafue Gorge v a r y e a c h y e a r between 600 and 1 6 0 0 km2, which
implies t h a t at least 600 km2 i s always inundated. In t h e r e a c h between Namwala
and Nyimba, fluctuation of levels in t h e Kafue Gorge have no effect, but this r e a c h
i s under t h e influence of regulation from t h e Itezhitezhi dam. Another e f f e c t of
t h e s e impoundments on t h e floodplain has been to greatly r e d u c e t h e amount of
land t h a t i s seasonally flooded. According to Lagler et aL. (1971), b e f o r e impoundments total a r e a of f l a t s liable t o flooding w a s 6604 km2, while t h e mean maximum
area inundated by high water from 1954 to 1964 w a s 4820 km2 and estimated mean
area flooded annually f o r a 4-month period w a s 2 8 3 3 krn2. Nowadays, Itezhitezhi
dam releases water at a relatively constant rate to a s s u r e a continuous supply to
t h e g o r g e power plant, thus reducing t h e height and e x t e n t of t h e floods in t h e
flats. Consequently, t h e western p a r t of t h e Kafue f l a t s is no longer flooded
(Sheppe 1985). In o r d e r t o p r e s e r v e some semblance of the floodplain ecosystems,
i t would b e necessary to a s s u r e r e l e a s e of sufficient water and at t h e r i g h t time
from Itezhitezhi but t h i s goal i s in conflict with t h e demand f o r e l e c t r i c power, and
t h e power agency wants t o minimize flooding in o r d e r t o r e d u c e water loss by evapotranspiration.
4.6. Water chemistry
The relationships between t h e Kafue r i v e r and i t s floodplain are numerous and
reciprocal.
Thus, r i v e r floods markedly influence biogeochemical c y c l e s which
o c c u r in floodplain soils, while in t u r n t h e s e p r o c e s s e s ( r e s p i r a t i o n , mineralization, etc.) influence t h e water quality of t h e r i v e r . The a v e r a g e conductivity,
which i s a good indicator of variations of r i v e r load, w a s measured at 1 7 2 pS with a
r a n g e of 1 1 0 to 260 pS ( S a l t e r 1985). Seasonal variation of conductivity p r e s e n t s
t w o maxima, o n e between August and October (230 pS) and o n e between J a n u a r y and
June (260 pS). The f o r m e r should b e d u e t o slow leaching of ions from soils during
t h e d r y season, while t h e latter should b e t h e r e s u l t of t h e flushing of t e r r e s t r i a l
n u t r i e n t s into a q u a t i c systems during flood periods. The a v e r a g e annual pH i s 7.9
but i t s values r a n g e between 6.7 to 9.3 depending on t h e hydrological cycle. A
s h o r t a g e of dissolved oxygen c a n o c c u r during floods d u e t o high biochemical
demand in oxygen by decomposition p r o c e s s e s ( S a l t e r 1979).
4.7. S e d i m e n t D e p o s i t i o n
Sediment deposition in Kafue r e s e r v o i r s i s not a n important problem because
t h e Kafue r i v e r flows through s e v e r a l floodplains (Lukanga, Busanga, Kafue f l a t s )
which act as sediment t r a p s . One of t h e consequences on t h e Kafue r i v e r downs t r e a m from t h e Kafue Gorge dam i s i t s l o w concentrations of suspended matter.
On t h e o t h e r hand, i t s ionic composition (dissolved m a t t e r ) i s v e r y high.
4.8. Ecological Aapects
The aquatic and t e r r e s t r i a l ecosystems of t h e Kafue f l a t s are closely i n t e r r e lated. The annual cycle of flooding and drying e x e r t s a profound influence on life
in t h e two biological systems. P r o c e s s e s are q u i t e common to t h o s e d e s c r i b e d f o r
t h e Kariba (Chapter 111, Section 2.8), b u t at a r e a l l y b r o a d e r s c a l e s i n c e i t conc e r n s a n area of a b o u t 5 6 0 0 km2. F u r t h e r m o r e , t h e p a r t i c u l a r hydrological c y c l e
of t h e Kafue r i v e r influences t h e s e processes. Thus, as t h e floodwaters r e c e d e
and the flood plain d r i e s up (May-September), t h e vast quantity of aquatic vegetation i s heavily grazed by herbivorous animals o r burned down (Figure 51). For
instance, t h e Kafue lechwre (Kobus leche k-ehsts),
a n endemic antilope well stu-
died by Rees (Rees 1978a-e), feeds on this stranded vegetation. When floods
occur, usually in December, t h e aquatic vegetation begins a period of rapid
growth. I t coincides with t h e reproduction period of most fish species (Lagler e l
C
'I*rrestri.l
and A q u a t i c
Reduction Increasing
T e r r e s t r i a l and A q u a t i c
Production Decreasing
------ -----HiBhest
*rreatrial Nutrients
Unshed I n t o Aquatic
Bystem d I n u n d a t e d
by Tlood Waters
\
-I
G r a s s e s Exposed by F a l l i n g Water Are
Crazed by I e c h v e and C a t t l e , a n d Are
m p l d Crorth of
r)puatic vegetation
Burned by F i r e s
Reproduction
Rapid
Growth Of
PI~hes
----
Fishes I n c r e a s i n g l y Vulnerable t o
P r e d a t i o n a s A v a i l a b i l i t y o f Food and
S h e l t e r Decreaaec
Rapidly Expanding
Aquatic E n v i r o m e n t
Aquatic
Environment
D i s s o l v e d Qrygen C o n d i t i o n s I ~ m d n g
T o l e r a b l e Only
Temperature F a l l i n g a n d
Photo R r i o d D e c r e a s i n g
T e m p r a t u r e e Riming d Photo P e r i o d Lengthening
bnths
J
Rainy Season
F
W
A
W
J
Dry B ~ m o n
J
A
S
W ~ t e rLevel
0
N
Rainy Seasc:
Figure 51. Generalized annual ecological cycle of t h e Kafue f l a t s (Lagler e l aL.,
1971).
The resultant young fishes find food and s h e l t e r in t h e vegetation in t h e fringe
zone of t h e floodplain margin. A s t h e flood crest begins to move downstream from
t h e flats, decomposition of organic matter inundated by t h e flood o r washed in by
runoff deoxygenates t h e waters from place to place. After t h e maximum flood i s
r e a c h e d , t h e aquatic ecosystem starts a period of slow contraction while primary
production d e c r e a s e s along with t h e falling temperature and diminishing photo
period (Lagler et al. 1971). By July or August t h e floodplain i s d r y and t h e surviving aquatic animals are concentrated in t h e lagoons and t h e r i v e r . This rough
description of a n a v e r a g e annual cycle in t h e Kafue f l a t s must not disguise one of
t h e most important p r o p e r t i e s of any ecosystem, which i s i t s variability from y e a r
t o year. Occasional extreme conditions may exclude long-lived species t h a t cannot
t o l e r a t e even one unfavourable y e a r , but on t h e o t h e r hand, variability of habitats
may permit a n increased variety of species to live in one area (Sheppe 1985). The
effect of t h e dams is t o r e d u c e t h e amount of y e a r to y e a r variations, but variation
will not b e eliminated altogether, because of variation in local rainfall; t h e Itezhitezhi r e s e r v o i r is not l a r g e enough t o buffer all variations in runoff from t h e
u p p e r drainage basin.
4.9. Fish and Fisheries
The Kafue fishery is o n e of t h e m o s t productive in Zambia. Of t h e 67 fish
species known to o c c u r in t h e Kafue fishery, 21 are of some commercial significance. Of t h e s e ten are more p r e f e r a b l e t o o t h e r s . They belong t o a Sarothero-
d o n s p , T i l a p i a s p Lapeo sp, C l a r i a s s p , Serronochromis s p , Schilbe s p and Heps e t u s s p (Chipungu 1981;Muyanga and Chipungu 1982). P r e - and post-impoundment
studies have been c a r r i e d out in o r d e r t o assess t h e e f f e c t of water regulation on
fish populations (Dudley 1974; 1979;Dudley and Scully 1980). I t i s c l e a r t h a t seasonal flooding of t h e Kafue flats provides a n extension to fish habitats f o r about
six months of t h e y e a r . Thus, t h e life cycle of fish i s closely r e l a t e d t o t h e flooding regime (Williams 1971).
A s i s shown in Figure 52,t h e high production of fish on t h e Kafue flats, and in
t u r n , commercial fishery, are dependent on t h e flooding regime. There h a s long
been a major fish industry on t h e flats, supported by high fish productivity in t h e
floodplain. The fishermen o p e r a t e largely f r o m small villages situated along t h e
r i v e r using gill-nets.
Few villages on t h e r i v e r banks o r mounds are sufficiently
elevated to b e habitable during t h e high water period. A s floods r e c e d e , fishermen and t h e i r families migrate along t h e r i v e r and adjacent lagoons in temporary
camps (Lagler et al. 1971). Table 36 gives t h e evolution of t h e fish yield on t h e
Kafue f l a t s from 1969 to 1978.
The slight increase in fish yield in t h e Kafue f l a t s i s due t o a significant
i n c r e a s e in fishing effort. In f a c t , t h e c a t c h p e r boat p e r night h a s dropped in
c o n t r a s t to t h e predicted increase in ichthyomass. Also, t h e r e have been a f e w
noticeable changes in relative abundance and weight of t h e commercially important
species. The m o s t significant change w a s t h e decline in catches of Sarotherodon
m a c r o c h i t , R l a p i a r e n d a l l i , and R l a p i a s p a r r m a n i i (Muyanga and Chipungu
1982). Fish yield in t h e Kafue f l a t s w a s and still remains a n important p a r t of t h e
.r..c.
.-.I,.
C".l.<l..
,".tr"-.-.
..
#""..,.B,,t,
DO.ll...
..
I...
Figure 52. Some interrelationships of factors of fish production in the Kafue flats
(Lagler et al.,1971).
W L e 36. Evolution of fish yield in Kafue flats (Chipungu 1981).
Year
1969
1970
1971
1972
1973
Fish yield@)
5728
5558
8247
7874
6289
1974
1975
1976
1977
1978
fish production of t h e Republic of Zambia. The fish industry i s actually one of t h e
main reasons why t h e Kafue f l a t s seems t o be a n a t t r a c t i v e area f o r t h e population
(Sheppe 1985). Considering t h a t fish i s a staple food in Zambia and t h a t t h e
fishery provides many jobs, this activity should b e seen as a n important f e a t u r e in
t h e region.
4.10. Other U s e s
Other uses are actually developed or being developed in t h e Kafue flats. A s
shown previously, about 15 m3/s are available from t h e Itezhitezhi f o r o t h e r uses.
I t a p p e a r s t h a t irrigation r e q u i r e s one l i t r e p e r second and p e r h e c t a r e . This
means t h a t given t h e p r e s e n t restrictions, only 15000 h a can b e i r r i g a t e d in t h e
Kafue flats. In Zambia, t h e most important c r o p s grown under irrigation are wheat
and sugar cane, potatoes and vegetables. Table 37 gives a seasonal cropwater
requirement f o r i r r i g a t e d c r o p s .
Table 37. Seasonal c r o p water requirement f o r i r r i g a t e d c r o p s in Zambia (Kicle
1982).
I
Crop
Sugar cane
Wheat
Citrus
Potatoes
Vegetable
Seasonal water (mm)
300-500
250-450
Among o t h e r s , t h e m o s t important agricultural p r o j e c t in t h e Kafue flats i s
t h e Nakambala s u g a r estate in t h e southeast edge of t h e flats just off t h e floodplains.
I t occupies some 10300 h a and depends largely on irrigation (Sheppe
1985). From planting to cutting t h e c a n e plant, i t r e q u i r e s about 2000 mm of water
f o r i t s optimum growth (Pike 1982). Of this, 750 mm i s assumed to b e normally provided by rainfall and t h e remaining 1 2 5 0 mm must b e supplied by t h e irrigation
system on t h e Kafue r i v e r itself.
Concerning t h e livestock in t h e Kafue flats, Bingham (1982) r e p o r t e d t h a t
about 250 000 c a t t l e depend on t h e flats for d r y season grazing between August and
November. The r i s e of water level with t h e r a i n s in December forces t h e c a t t l e off
t h e flats. These h e r d s belong to t h e Tonga people to t h e south and t h e Ila in t h e
west.
Both numbers a n d distribution of l a r g e mammals in t h e f l a t s h a v e been g r e a t l y
r e d u c e d d u r i n g t h i s c e n t u r y , especially t h e lechwre, b u t also buffalo, z e b r a and
eland among o t h e r s . In o r d e r to s t o p t h i s d i s a p p e a r a n c e of mammals, Lochinvar
Ranch was d e c l a r e d a National P a r k in 1972,a n d t h e Blue Lagoon Ranch achieved
t h e same s t a t u s in 1973 (Mwenya a n d Kaweche 1982). Wildlife utilization in t h e
Kafue f l a t s h a s e n c o u r a g e d tourism as w e l l as subsistence a n d r e c r e a t i o n hunting.
The small amount of accomodation available a n d t h e lack of c a t e r i n g are t h e major
limiting f a c t o r s f o r tourism in t h e Kafue f l a t s .
Chapter TV: Consequences of Impoundments on the
Zambezi Drainage Basin
The preceding description of t h e impoundments on t h e Zambezi basin are
mainly a b o u t man-made lakes. The Zambezi r i v e r system h a s gradually changed due
to human i n t e r f e r e n c e s , and h a s r e a c h e d t h e second s t a g e of a n t h r o p i c development according to David (1985).
Here, t h e n a t u r a l runoff system gradually
changes t o become more regulated. Thus, i t h a s been said t h a t almost 570 km of
t h e Zambezi i s regulated by man-made lakes. Moreover, t h e s t o r a g e capacity along
t h e r i v e r system h a s increased tremendously. These r e s e r v o i r s were built f o r t h e
sole p u r p o s e of generating hydropower. Nevertheless, they a f f e c t many d i f f e r e n t
substances of t h e fluvial ecosystem. In t h e previous c h a p t e r , t h e consequences of
dam impoundments on t h e r e s e r v o i r s themselves, particularly with r e s p e c t t o limiting secondary uses h a v e been described. However, downstream e f f e c t s on t h e s e
huge r e s e r v o i r s must also b e considered. Thus, a quick overview of t h e main challenges t h a t have emerged will b e given in t h e following sections.
1. C o n s e q u e n c e s Downstream from the Kariba Dam
1.1. M a n a Pools Wildlife R e s e r v e
Within Zimbabwe, t h e r e i s a wildlife estate covering virtually a l l of t h e valley
between t h e e a s t e r n end of Lake Kariba and t h e Mozambique b o r d e r , occupying
some 1 2 0 0 0 km2 (Figure 53). The Government of Zambia h a s d e c l a r e d a l a r g e a r e a
o n i t s s i d e of t h e Zambezi r i v e r as a national p a r k , which t o g e t h e r with t h e Zimbabwean p a r t c o v e r 1 6 8 0 0 km2. This area i s particularly r i c h in l a r g e mammals
(Du Toit 1984, 1985) t h a t f e e d on vegetation growing on t h e Zambezi floodplains, a n
area providing food, s h a d e and water f o r game animals and b i r d s during t h e d r y
season. Both t h e hydrologic a n d t h e biological cycles of t h e floodplain are similar
to t h a t of t h e Kafue swamps (Chapter 111, Section 8).
N O ~ I D Park
~O~
F w e 53. Mid Zambezi wildlife area (DuToit, 1985).
The Kariba impoundment h a s d i s r u p t e d t h e ecological equilibrium, a n d t h i s
may b e a t t r i b u t e d to d i f f e r e n t r e a s o n s (Figure 54).
- D e c r e a s e in flood p e a k s
in t h e f l o w a n d d u r a t i o n , a n d out-of-season floods a f f e c t
t h e r i c h n e s s a n d biomass of t h e vegetation. This d e c r e a s e in flood d u r a t i o n c a u s e s
game animals to p a s t u r e on t h e floodplain, which in t u r n also a f f e c t s t h e vegetation
biomass.
- The t r a p p i n g of
sediment in t h e Kariba r e s e r v o i r provides a silt-free o u t l e t for
water which c a u s e s a n i n c r e a s e in bank erosion.
- The o v e r p a s t u r e b y game on r i p a r i a n
vegetation influences t h e effectiveness of
t h e p l a n t s o n t h e banks in reducing erosion.
F o r instance, a c c o r d i n g to Guy (1981) between 1954 and 1973,1030 h e c t a r e s
were lost to e r o s i o n o v e r a d i s t a n c e of a b o u t 40 km. This s t r e t c h r e p r e s e n t s only
10 p e r c e n t of the total length of t h e bank downstream from Lake Kariba to t h e
Zimbabwe-Mozambique
b o r d e r . I t may t h e r e f o r e b e assumed t h a t many more hec-
tares of soil were e r o d e d outside t h i s area. p a r t i c u l a r l y downstream from Mana
Pools.
This wildlife area i s a n important s o u r c e of f o r e i g n c u r r e n c y for Zimbabwe,
a n d conservation i s of utmost ecological and economic i n t e r e s t t h a t must b e cons i d e r e d from t h e point of view of r e s e r v o i r o p e r a t i o n management.
1
I
QUALITY
I
BANKS
EROSION
K A R I B A OUTFLOW
I\
I
decrease
E I
VEGETATION
I
(
QUANTITY
1
Floods less in
surface and duration
II
decrease of
resources
Figure 54. Consequences of Kariba outflow on t h e ecological equilibrium of Mana
Pools floodplain.
1.2. Inflow at Cahora Bassa
The Kariba outflows constitute about 75 p e r cent of t h e total inflow into t h e
Cahora Bassa r e s e r v o i r . The discharge capabilities of t h e Cahora Bassa dam and
i t s s t o r a g e capacity are too small t o p r o t e c t t h e dam from overtopping even floods
below t h e design flood magnitude. On t h e o t h e r hand, flood generation within t h e
Cahora Bassa drainage basin i s extremely i n t r i c a t e and difficult to p r e d i c t accurately. Accordingly, t h e water level in t h e Cahora Bassa r e s e r v o i r must be drawn
down heavily each y e a r during t h e d r y season in anticipation of t h e next flood in
t h e rainy season. Moreover, t h e presence of t h r e e major impoundments within t h e
catchment, each with i t s own specific operating rules, i s a n additional complication. In fact, t h e Kariba r e s e r v o i r operations seem to have considerable influence
on t h a t of t h e Cahora Bassa. The upstream regulation affects t h e flood discharges
downstream. In t h e d r i e s t y e a r s , regulation is likely to reduce t h e annual maximum
t h r e e month floods since natural floods will b e s t o r e d in t h e upstream r e s e r v o i r s
to provide electricity at a later date. In t h e wettest y e a r s , assuming t h e r e s e r voirs are o p e r a t e d according t o a reliable flood r u l e c u r v e procedure, t h e annual
maximum three-month floods will also be reduced, t h e amount being equal t o t h e
total s t o r a g e capacity of t h e r e s e r v o i r s ( o r flood control
- Bolton, 1983). Between
these extremes, t h e effect will depend on t h e operating policy adopted f o r t h e
upstream r e s e r v o i r s . For instance, t h e Kariba p r o j e c t was not y e t operated consistently according t o flood r u l e c u r v e procedures between 1971 and 1980, as
shown in Table 38.
Table 38. Reservoir levels at Kariba on February 1 (Bolton, 1983).
Year
Reservoir level (m.a.s.1.)
Flood s t o r a g e provided (103m3)
1971
484.2
27.6
1972
483.7
30.3
1973
483.3
32.2
1974
484.6
25.4
1975
486.6
14.9
1976
484.7
25.0
I t h a s been suggested by Rendel, Palmer and Tritton (Bolton, 1983) t h a t
3 0 . 8 . 1 0 ~ mshould
~
b e t h e flood s t o r a g e on February 1 ( r e s e r v o i r level a t 483.6
9 3
m.a.s.1.) and 13.7.10 m ( r e s e r v o i r level at 486.8 m.a.s.1.) on March 1. I t a p p e a r s
t h a t a joint operating policy i s necessary in o r d e r t o optimize hydroelectric power
generation by each power plant a s well as t o sustain ecological equilibrium in t h e
valleys.
2. T h e C o n s e q u e n c e s Downstream from the Kafue Gorge Dam
The consequences downstream from t h e Itezhitezhi dam have already been elaborated in t h e previous c h a p t e r . Concerning t h e Kafue Gorge dam, t h e consequences downstream from t h e impoundment seem to be similar to t h a t of t h e Kariba.
Thus, i t is essential t h a t a joint operating policy which should involve managers of
t h e Kariba, Itezhitezhi, Kafue Gorge and Cahora Bassa r e s e r v o i r s as well as t h e
wildlife conservationist b e developed in o r d e r t o sustain t h e s e v e r a l p u r p o s e s at
s t a k e on t h i s p a r t i c u l a r r e a c h of t h e Zambezi.
3. The Consequencea Downstream from the Cahora Bassa Dam
3.1. Ecological Consequencea
The s h o r t a g e of flow downstream from t h e dam during t h e filling p h a s e h a s
a l r e a d y been mentioned (Davies 1975a,b). Moreover, e r r a t i c management until now
h a s not allowed a n y kind of ecological r e s t o r a t i o n in t h e fluvial system downstream
from t h e r e s e r v o i r . The annual floods b e f o r e t h e Cahora Bassa impoundment r e j u venated t h e e n t i r e r i v e r i n e and floodplain system with water, f r e s h s i l t and
nutrients. A t p r e s e n t , floods t h a t are buffered are even worse, f o r t h e y o c c u r o u t
of season disrupting t h e whole n a t u r a l c y c l e of flood a n d e b b , even if a f r e s h
d i s c h a r g e of 7.109m3 o v e r a p e r i o d of 1 2 d a y s in F e b r u a r y e a c h y e a r is planned.
On t h e o t h e r hand, sediment t r a p p e d in t h e r e s e r v o i r is n o longer available t o sustain t h e maintenance of t h e Zambezi d e l t a , a t r i a n g u l a r area of 1 8 0 0 0 km2 with a
sea f r o n t a g e 120 km long. The Marromeu Buffalo R e s e r v e c o v e r s 1 6 0 0 km2 of t h e
delta. The evolution of t h i s area i s v e r y close to t h a t of t h e Mana Pools (Section
1.1): d e c r e a s e of flood p e a k , overgrazing, l e s s input of s i l t from t h e r i v e r , and
erosion. In addition, t h i s d e c r e a s e in sediment deposition as well as t h e d e c r e a s e
in r i v e r flow n o longer h i n d e r intrusion and salinization by sea water in t h e alluvial soils devoted mainly to s u g a r c a n e a g r i c u l t u r e .
This d e c r e a s e of t h e
suspended m a t t e r load in t h e Zambezi r i v e r could a l s o h a v e a negative e f f e c t on
t h e fish yield along t h e c o a s t .
3.2. Navigation and Irrigation
I t was assumed t h a t t h e Cahora Bassa p r o j e c t should not only provide
hydroelectricity but a l s o water f o r a g r i c u l t u r a l p u r p o s e s downstream from t h e
r e s e r v o i r a n d a 500 km long navigable s t r e t c h . A t p r e s e n t , navigation o n t h e
lower Zambezi is r e s t r i c t e d to a few f e r r i e s . P r o s p e c t s of f u t u r e expansion of
navigation depends p a r t l y on t h e minimum g u a r a n t e e d flow of t h e r i v e r (1600 m3/s
from Cahora Bassa), but more on t h e morphological c h a r a c t e r i s t i c s of t h e channel
(Bolton, 1983).
Today v e r y l i t t l e w a t e r i s e x t r a c t e d f r o m t h e Zambezi f o r i r r i g a t i o n . In f a c t ,
f o r t h e f o r e s e e a b l e f u t u r e , w a t e r i s unlikely to b e a limiting f a c t o r in t h e possible
expansion of i r r i g a t e d a g r i c u l t u r e .
Chapter V. Conc3usions
In t h i s r e p o r t , t h e Zambezi d r a i n a g e basin c o n s t i t u t e s o n e entity, and i t i s
only f o r t h e s a k e of p r e s e n t a t i o n t h a t i t i s s e p a r a t e d into d i f f e r e n t topics. It i s
shown t h a t t h e r e are many i n t e r r e l a t i o n s h i p s amongst t h e r i v e r system components, s u c h as hydrological, biological, chemical, a n d anthropogenic, whatever
t h e p e r c e i v e d scale may b e . In a l a r g e international r i v e r basin such as t h e Zambezi, t h e p r o s p e c t s of long-term i n t e g r a t e d development may a p p e a r a t t r a c t i v e ,
b u t given t h e available r e s o u r c e s and t h e complexity of t h e issues, s h o r t t e r m
expediency a n d narrowly defined objectives are likely to dominate policy formulation in t h e f o r e s e e a b l e f u t u r e .
The proposed new impoundments on t h e Zambezi r i v e r c o n c e r n mainly power
plants (Figure 55). Taking into a c c o u n t t h e a c t u a l power g e n e r a t i o n c a p a c i t y and
t h e a c t u a l demand f o r i t , i t seems more r e a s o n a b l e even from t h e h y d r o e l e c t r i c i t y
generation p e r s p e c t i v e to t r y to optimize e n e r g y o u t p u t from t h e a l r e a d y existing
hydropower plants. In such circumstances, t h e r e i s a n u r g e n t need f o r f u r t h e r
studies of existing p r o j e c t s to focus attention on t h e impacts of l a r g e dam p r o j e c t s
and to e n a b l e more r e a l i s t i c estimates of t h e t r u e costs a n d benefits.
11250
A1 E A C H DAM SITE
- IWOMWI
E
Il2LO M W I
11soo
- UrnMWl
UlLOYElREI
E
;
m',g,.,;cr
m
m
0
ID
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UD
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r'
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I s
In=
Esd..easrr
,Linbrbur , Sep.
b ! ~. c u l c e b C c r ~ t
--
1980.
-
-.*,
Source: FolcAo (1Yb3)
---
Figure 55. Proposed hydroelectric development on the Zambezi river (Bolton,
1983).
- - .- . -.
1
REFERENCES
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t h e mid-Zambezi valley of Rhodesia. B o l . Conserv. 2(3)189-196.
Austin, R.H.F. (1968) International legal a s p e c t s of t h e Kariba p r o j e c t . In: N .
Rubin a n d W.M. Warren (Eds.) "Dams in Africa", 146-157.
Bailey, R.G. (1985) The f a c t o r of s c a l e in ecosystem mapping. E n v i r o n . Manag.,
9(4)271-276.
Balasubrahmanyam, S. a n d Abou-Zeid, S.M. (1982a) The Kafue r i v e r hydro-electric
development. Proc. Nut. Sem. o n E n v i r o n m e n t a n d Change: t h e Consequences of Hydroelectric Power Development o n t h e U t i l i z a t i o n of t h e K-e
R a t s , University of Zambia, Lusaka, 1978, 31-33.
Balasubrahmanyam, S. a n d Abou-Zeid. S.M. (1982b) Post-Itezhitezhi flow p a t t e r n of
t h e Kafue in t h e Kafue flats region. Proc. Nut. Sem. o n E n v i r o n m e n t a n d
Change: t h e Consequences of Hydroelectric Power Development o n t h e U t i l i z a t i o n of t h e K-e
R a t s , University of Zambia, Lusaka, 1978, 63-67.
Balek, J. (1977) Hydrology a n d w a t e r r e s o u r c e s in t r o p i c a l Africa. Development
i n Water Science, 8 , E l s e v i e r , Amsterdam, 208p.
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In: W.C. Ackermann, G.F. White and E.B. Worthington (Eds.) "Man-Made Lakes:
T h e i r Problems a n d Environmental Effects", 620-628.
White, W.R. a n d Bettess, R. (1984) The feasibility of flushing sediments t h r o u g h
r e s e r v o i r s . In: "Challenges in African Hydrology a n d Water Resources", IAHS
Publications, No. 144, 577-587.
Williams, R. (1971) Fish ecology of t h e Kafue r i v e r a n d floodplain environment.
fish. Res. Bull. Zambia, 5, 305-330.
Wilson, B.H. and Dincer, T. (1976) An introduction to t h e hydrology a n d h y d r o g r a phy of t h e Okavango d e l t a . In: Proc. of the S y m p o s i u m on Okavango Delta
a n d i t s F'uture U t i l i z a t i o n , Gaborone, Botswana, 33-47.
Annex I. Technical Details o n t h e Main Impoundments of t h e Zambezi D r a i n a g e
Basin (Bolton, 1983).
Karibn
-
C d ~ o r al{nss;~
Cmstructim cam~nced
lrprnmdinq c-nced
.K-; , luc
~ L . , I I I D;,m
Kafue Iiorqe I)ouq
-11 I?.~II
IJCI. 1 ' 1 7 ~ ~
Sept 1969
Dec 1974
.July 1955
k c 1950
1967
1970
cupola arch
concrete
cupola arch
concrete
gravity
earth-rockri I 1
197 1
D m details
Type
ktcrial
I h x ~ n u nh e i c p t (m)
Crest l e n g t h (rn)
Volune o f d m (m' x 10')
Hock excevation underqround (m' x 1 0 ' )
0;*n c r e s t a l t i t u d e (10 1l.D.
5+illway c a p a c i t y ( m 9 / s )
l(33
X)3
550
1100
331.0
13950 ( a t 329 m)
131
50
633
375
975
1200
580 + 320.
489.5
981.5
9500 ( a t 409 m) 4250
y r : ~ vtl )
e a r t l n - r u ~ k 11
f I
0 ,'
1tNln
U5llfl
!I
lfl11.5
421li1
--
Reservoir d e t a ~ l s
Storoqe c a p a c i t y : l l v e
( d m 10'1
dead
r lood
total
Surfoce m e n , normal o p e r a t i o n (km')
H r ~ x i n ml e n q t h ( k a )
Muxinun p r o j e c t e d drawdown (rn)
-
v i ~ l wnot known;
iw1udt:s
**
tlie N o r t h Il;lnk Powcr 5 l ; t t l o n ;
includes p a r t i a l l n u n d a l ~ o nu r 1111 K.tlue f l i ~ l s .
Knr~ba
-
Ci~bor:i flason
K;tl
KofueraryeDano
tor
! k l d ~ L t , ~Dam
h~
Ilydrolo~~y
Cutchnent oren ( k a g x 1 0 ' )
Q111rnr.. ue;m amon1 i n f l o w (m' x l f l ' )
4pl1rou. min. ret)ul;~ted flow (III'/S)
l k ~ x i m u nunretjuleted flow (m3/s)
t l i r ~ i r m nunrequlated flow (cnS/s)
1OUU
84 •
2lOfl
3flD00
Power G e n e r a t ~ m
Cahora nosria South
l u r b l n e Lype
Nupber o f sets
Connerci;~l 1-r
oveilnble
H i ~ t e dt r r t ~ u tp e r s e t (Md)
l b r i m w n t u r b i n e d ~ s c h a r g e(rn1/o)
( C l x ~ u mqrovs head (m)
IkneraLor output n t 50 llz (kV)
Power f a r t o r
V e r t i c a l Francis
b i n r i p a l t r ; ~ n s m i s s i m v o l t : ~ q e (kV)
I'rojected cnertly p o L e n t i a l (IWh/yr)
Projected
u l t i o t c upar*t¶
- v i ~ l u en o t Lmo*n;
6 50
52
1500
160n0 ( i n 1950)
2Un ( i n 1949)
am
5
K n r i l ~ aSouth
Knri11.1 N t , r t l ~
V e r t i c a l Franc15
4
1976
150
200
1on
10
0.9
6
March 1977
Oec 1959
415
452
1 2U
111
0.U5
111"
140
110
10
0.9
f>33 (d.c.)
10.5 (99:. p r o b )
20.5 (95% p r o b )
3non
a l l m ~ n qf o r c v a ( ~ t ) r a t l o na t Kar~b:>;
150
10
180
2400
10
**
330' (a.c.
over 111
-
1 5 ~ 0 i8on
o r i q i n n l l y r a t e d e t 100 IM
V r r t l c a l Francis
4
2
Annex 11. (1) Details of the Cahora Bassa Dam (Bolton, 1983; Bernacsek
and Lopes, 1984).
JSE
Annex 11. (2) Hydraulic c e n t r a l c i r c u i t and technica! d e t a i l s of t h e
C a h o r a Bassa (Bolton, 1983;Bernacsek a n d Lopes, 1984).
HYDRAULIC CENTRAL CIRCUIT
JiEX
TECHNICAL DATA FOR CAHORA RASSA DM Ah3 SOUTH SHOOE POhTR STATION
Geographical p o s i t i o n
15035'S; 32044'E
Date of c l o s u r e
5 December 1974
Dam v a l l
C r e s t h e i g h t above r i v e r b e d
Uaximum t h i c k n e r r of f o u n d a t i o n
Uinimum t h i c k n e s r of v r l l ( n e a r c r e r t )
Volume of c o n c r e t e ured
Flood ( r l u i c e ) g a t e 1
Number
D i r c h a r ~ ec a p a c i t y of r i n g 1 8 s l u i c e ate
( a t 326.00 m.a.r.1.)
T o t a l d i s c h a r 8 e c a p a c i t y of e i g h t r l u i c e g a t e r
( a t 326.00 m.a.r.1.)
Spill (flap) gate
Discharge c a p a c i t y ( a t 326.00 m.a.s.1.)
Turbines
Number
Generating c a p a c i t y of r i n g l e t u r b i n e
Norm1 t o t a l g e n e r a t i n g c a p a c i t y (4 turbine11
Haximum t o t a l g e n e r a t i n g c a p a c i t y (5 t u r b i n e s )
Design rpeed of t u r b i n e
C r o r r head t o t u r b i n e r
Flow t o s i n g l e t u r b i n e
Normal t o t a l t u r b i n a t e d flow (4 turbine11
Haximum t o t a l t u r b i n a r e d f l w (5 t u r b i n e r )
T o t a l m.aximm d i s c h a r g e c a p a c i t y of dam and power
s t a t i o n ( a t 326.00 m.a.s.1.)
5 (4 o p e r a t i o n a l + rtandby)
415 nu
I 660 Wv
2 075 UW
107.1 RPM ( c o n r t a n t )
103.5 m
L(0 m'lrec
1 840 m J / r c c
2-300 3I n e c
Annex 11. (3.1) Hydrological data for the Cahora Bassa 19'74-19'76 (Bernacsek and Lopes, 1984).
'(~96~;
'sad07 pue yasaeu
'I1 XauuV
-JW)
8,!,6T-L,!,61
EssE€i eJoqe3 ar17 JoJ e7eQl e ~ ! a o l o J Q ~ (2'~)
H
Annex 11. (3.3) Hydrological data for the Cahora Bassa 1979-1980 (Bernacsek and Lopes, 1984).
nmotmIcu
DATA
ra wau
M:sr u : s t n v o ~ ~
I919
Annex 11. (3.4) Hydrological data for the Cahora Bassa 1981-1982 (Bernacsek and Lopes, 1984).
W T D R O U Y l C U DATA TQ
r.brwv
Ib
114.11
1 a81
I
lIb.70
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Ib
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2 051
m
111.C.
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uin
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WrYI U S S A NSCRVOIR
1981
I
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1.0
1.b5
1.0
1 ,&I
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0.09
1.0
1.59
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12.61
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99'91
09'91
70'91
05'51
(=)
750 C
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716 Z
568 Z
@I82
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599 Z
I65 2
115 Z
577 2
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591 2
860 Z
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