S.-Afr. Tydskr. Geo1.1988,91 (3),346-349
346
Plant distribution and the evolution of the major river systems in southern Africa
A.E. Moore
AMPAL, Box 10072, Gaborone, Botswana
Accepted 28 April 1988
Geomorphological evidence indicates that the upper reaches of the Zambezi River were originally linked to the
present Limpopo River. Disruption of the Greater Limpopo by Plio-Pleistocene upwarping resulted in the
formation of the modern Zambezi and Limpopo river systems. It is suggested that plant species, presently found
as paired isolated populations centred on these two rivers, formerly had a more extensive distribution along the
Greater Limpopo river system. Following severance of the link between the Limpopo and upper Zambezi, the
earlier distribution ranges contracted under pressure of Pleistocene climatic change.
Geomorfologiese gegewens dui daarop dat die bolope van die Zambezirivier aanvanklik verbind was met die
huidige Limpoporivier. Ontwrigting van die Groter-Limpopo deur Plio-Pleistoseenopbuiging het tot die
vorming van die moderne Zambezi- en Limpoporiviersisteme gelei. Daar word gesuggereer dat plantspesies,
wat tans as stelle gelsoleerde populasies, gekonsentreerd rondom hierdie twee riviere voorkom, vroeer 'n meer
uitgebreide verspreiding langs die groter Limpoporiviersisteem gehad het. Gevolg deur afsnyding van die
verbinding tussen die Limpopo en Bo-Zambezi, het die vroeere bestek van die verspreiding nouer getrek onder
druk van Pleistoseenklimaatsverandering.
Introduction
Numerous southern African plant species occur in
isolated and often widely separated populations (CoatesPalgrave, 1977). Such isolated commumtles are
generally considered to be remnants of an originally
more extensive distribution range, which contracted into
localized favourable habitats under pressure of climatic
change (Levyns, 1964; Wild, 1968). Where habitat
preference is well established, it should be possible to
infer earlier distribution ranges from an understanding
of former climatic regimes and landscape evolution. This
could be done with greatest confidence where a number
of plant species display similar characteristic distribution
patterns.
Distribution data summarized by Coates-Palgrave
(1977) indicate that numerous southern African plant
species occur in isolated paired populations centred on
the Limpopo and Zambezi rivers respectively. Examples
of species showing such a distribution pattern are
illustrated in Figures l(a-g). Some of the species
occurring on these two drainages may in addition be
found in isolated populations in northern Namibia (e.g.
Figures 1a and 19). This paper explores possible
implications of these and inferred related distribution
patterns (Figures 1h-1j) regarding evidence for the
geomorphic evolution of the Limpopo and Zambezi
rivers.
Evolution of the Limpopo and Zambezi river
systems
The major southern African rivers are shown in Figure
2. The Zambezi and Limpopo, separated by some 500
km at their closest point of approach, are the major
eastward draining systems. However, their present
courses are considered to have been established
comparatively recently. Bond (1963, 1975) has
postulated that the Cuito-Okavango, Cuando and upper
Zambezi were formerly tributaries of the Limpopo. This
is supported by several lines of evidence:
1. Diamonds and kimberlite indicator minerals have
been recovered in the upper Motloutse, a tributary of
the Limpopo (Figure 3) (Willis, 1960; De Beers
Prospecting (Pty) Ltd., 1972). A former link between
the Okavango and the Motloutse, as suggested by
Bond, would have traversed the Orapa kimberlite
field to the west of the present watershed (Figure 3).
This could account for the alluvial stones and
kimberlitic heavy minerals in the Motloutse.
2. Bond (1963, 1975) noted that the catchment of the
modern Limpopo is smaller than that of the Zambezi,
and lies in an area of relatively lower rainfall than the
latter drainage system. Despite this, the Limpopo has
incised a deeper valley than the Zambezi. In addition,
the Zambezi delta is poorly developed relative to the
present size of the river system, and contrasts strongly
with the well-developed delta at the mouth of the
Limpopo (Martin, 1984). This points to a formerly
more extensive Limpopo drainage, and relatively
recent development of the modern Zambezi system.
3. The sharp deflection of the upper Zambezi from a
south-easterly to an easterly course just north of the
Botswana border (Figure 2) is suggestive of river
capture. Bond (1975) postulated that a southeastward continuation of the present upper Zambezi
course may have formerly linked this drainage to the
Limpopo. This may have been via the Motloutse,
which has a gorge larger than would be expected on
the basis of the present-day river load. Alternatively,
the connection may have been via the Shashi, which is
roughly colinear with the course of the upper Zambezi
(Figure 3). The former link would have been severed
by river capture of the upper Zambezi by the middle
Zambezi.
4. Modern distribution of fish fauna could be more
readily explained if the Limpopo and upper Zambezi
were formerly linked (Bond, 1975; Gaigher & Pott,
1973; Bowmaker et al., 1978).
5. The orientation of faint fossil drainage lines in
347
S.Afr.J.Geol.I988,91(3)
iJl_-J~n
,
,
~
:
I
~
!l-/-_J
,
!.{
(>
/, ... j
i
Figure 1 Distribution of some southern African plant species (shown in black). Arrow heads denote localities of minor outlier
populations. In Figures la-lj, solid lines show the courses of the Zambezi (upper) and Limpopo (lower) rivers. Solid lines in
Figures lk-ll show the modern courses of the Orange (upper) and Olifants (lower) rivers. Thin dashed lines are political
boundaries. Plant species represented are: a: Commiphora merkeri; b: Xanthocercis zambesiaca; c: Excoecaria bussei; d:
Hippocratea parvifolia; e: Milletia usaramensis; f: Cordia goetzei; g: Gyrocarpus americanus; h: Commiphora edulis; i: Strychnos
cocculoides; j: Adansonia digitata; k: Metrosideros angustifolia; 1: Carissa haematocarpa.
rI
r-
I
J
80
i
k1\
:I II
i ~'V
/
,,/
SA
I
Figure 2 Distribution of major rivers in southern Africa (solid
lines) and fossil drainage courses (dashed lines). 1: Zambezi;
2: Limpopo; 3: Cuando; 4: Chobe; 5: Cuito-Okavango; 6:
Shire; 7: Orange; 8: Olifants; 9: Cunene. Dot-dash line is the
Kalahari-Rhodesia upwarp axis of Du Toit (1933). Thin
dashed lines are political boundaries. Bo = Botswana; Ma =
Malawi; Mo = Mocambique; Na = South West Africa/
Namibia; SA = South Africa; Zi = Zimbabwe; Za = Zambia.
northern Namibia (Figure 2) suggests that these might
be relict tributaries of the Okavango, and thus
possibly also part of the former Greater Limpopo
drainage system.
Figure 3 Detail of drainages related to the former Greater
Limpopo (solid lines). 1-4 as in Figure 2. 5: Cuito; 9:
Okavango; 10: Boteti; 11: Motloutse; 12: Shashi. Dot-dash
line as for Figure 2. Thin dashed lines are political boundaries.
M = Makgadigadi Pans complex. Remaining abbreviations as
for Figure 2. Solid diamond shows the position of the Orapa
kimberlite field.
The age of disruption of the former greater Limpopo is
not well established, but can be inferred from indirect
evidence:
1. Du Toit (1933) has identified the watershed in eastern
Botswana as an epeirogenic upwarp axis of Plio-
348
Pleistocene age. Although this view is disputed by
Lister (1979), it is supported by the recovery of
diamonds and other kimberlitic heavy minerals in the
Motloutse River because,
despite extensive
prospecting in eastern Botswana, the only known
possible source rocks are the Orapa kimberlites to the
west of the drainage divide (Figure 3) (De Beers
Prospecting Ltd., 1972). The flexure identified by Du
Toit would have crossed the proposed course of the
palaeo-Zambezi, and might therefore have been
responsible for the reorganization of the Greater
Limpopo drainage system. It should be noted,
however, that Plio-Pleistocene uplift followed axes of
Miocene warping (King, 1978; Partridge & Maud,
1987). Piracy of the upper Zambezi by the middle
Zambezi may therefore have been initiated by the
earlier uplift (Lister, 1979).
2. Kalahari sand, with conspicuous seif dunes developed
at the surface, blanket much of the BotswanaZimbabwe border, and thus the hypothetical former
line of the Zambezi. The age of these sands is not well
established. A lower to mid-Pleistocene age is
tentatively given by Du Toit (1954), while Mabbutt
(1957) and Partridge & Maud (1987) consider them to
be of late Tertiary vintage.
3. Capture of the upper Zambezi by the middle Zambezi
would account for the rapid headward erosion that
formed the 101 km gorge below the present Victoria
Falls. The lower Pleistocene age for the development
of the lower reaches of the gorge (Derricourt, 1976)
would then be expected to coincide closely with the
time of river capture.
Collectively therefore, the available evidence suggests
an upper Tertiary - lower Pleistocene age for the
severance of the Zambezi-Limpopo link.
Discussion
Wild (1968) suggested that a 50% increase in rainfall
during Quaternary pluvials may have enabled riverine
species to expand their ranges to form a more or less
continuous forest at lower altitudes throughout much of
tropical Africa. If so, isolated populations of such
species on the Zambezi and Limpopo drainages may
merely reflect contraction of earlier more extensive
distribution ranges into favourable present habitats. This
possibility is difficult to rule out. However, Axelrod &
Raven (1978) maintained that it is unlikely in view of the
dry winters expected during the pluvial episodes.
Implicit in their argument is the assumption that plant
species showing a present-day habitat preference for the
hot, low-lying Limpopo and Zambezi valleys are
strongly dependant on ecological factors such as a
relatively shallow water table in the vicinity of these
rivers. If so, rainfall would play a secondary role in
controlling their distribution ranges. Range expansion
would then be expected to occur most readily along the
major river courses in response to changing climatic
conditions.
The existence of an extensive Greater Limpopo
drainage system that included the upper Zambezi,
S.-Afr.Tydskr.Geol.1988,91(3)
Okavango and northern Namibian drainage lines, and
possibly the Shire - mid-Zambezi, would have provided
convenient pathways for migration of riverine species.
The plant distribution patterns illustrated in Figure 1
could then be readily explained as relicts of originally
extensive riverine populations isolated by severance of
the Zambezi-Limpopo link. They therefore support
Bond's model for the evolution of the major eastward
draining rivers in southern Africa.
A number of tree species found along the Limpopo
and Zambezi rivers also occur in a belt straddling the
Zimbabwe-Botswana border (Figures 1h-1j). This is
coincident with a continuation of the south-east line
followed by the upper Zambezi before veering sharply to
the east (Figure 3). It is speculated that such distribution
patterns may reflect the course followed by the former
Zambezi-Limpopo link, possibly related to the ability of
these tree species to tap a deep aquifer in sedimentary
infill in the palaeo-river valley. This would favour a link
between the two rivers via one the Shashi tributaries
rather than the Motloutse. It may also have important
economic implications for farming development
currently being initiated in the dry north-east of
Botswana.
The inferred age (late Tertiary - early Pleistocene) for
severance of the Zambezi-Limpopo link sets a lower
limit for isolation of the Limpopo and Zambezi riverine
species. Such relatively recent separation is consistent
with the preservation of species integrity between the
paired populations. Contraction of former distribution
ranges would probably have been a response to
Pleistocene climatic changes.
Orange and Olifants rivers
Dingle & Hendy (1984) showed that a major
reorganization of the Orange River drainage occurred
during the Tertiary. In the early Tertiary, the lower
reaches of the Orange River followed a course
considerably to the south of the modern channel,
emptying into the Atlantic via the Olifants River mouth
(Figure 2). The disruption of the Olifants-Orange
palaeo-drainage is ascribed to Oligocene volcanic
activity by Dingle & Hendy (1984), whereas Partridge &
Maud (1987) suggested that the link between the two
westward-flowing rivers was severed during the
Miocene.
Data presented by Coates-Palgrave (1977) indicate
rare examples of isolated plant species occurring along
both the modern Orange and Olifants rivers (Figure
1k-ll). Such distribution patterns may reflect the former
link between the two westward-flowing drainages.
The Olifants River lies at low altitudes in a winter
rainfall region with relatively high precipitation. In
contrast, much of the course of the Orange River is
through an arid summer rainfall area at elevations above
1000 m. It is probable that these two drainage systems
have experienced differing climatic conditions since at
least the late-Cretaceous. In contrast, the Zambezi and
Limpopo are likely to have experienced broadly
comparable (savanna) climatic conditions over the same
S.Afr.J.Geol.1988,91(3)
period (Axelrod & Raven, 1978). Substantial long-term
differences in climatic regime, coupled with the
relatively early severance of the drainage link, would
account for the rarity of the isolated paired plant
populations along the courses of the two westwardflowing rivers.
Acknowledgements
Keith Simms of Seltrust (Botswana) Explorations and
Kandis Mclaughlin made possible a field trip to the
remoter parts of northern Botswana to discover where
the ancient Zambezi did not flow. Suggestions and
critical comments by Eugene Moll, Paul Shaw, Mike
Main and the two reviewers are gratefully
acknowledged.
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