Environmental Issues in the la Plata Basin
Carlos E. M. Tucci and Robin T. Clarke
Institute of Hydraulic Research
Federal University of Rio Grande do Sul
Porto Alegre – Brazil e-mail: Erro! A origem da referência não foi
encontrada.
ABSTRACT: The la Plata River basin is one of the largest in the world, with an area of
about 3 million km2. It includes parts of five countries (Paraguay, Argentina, Bolivia,
Brazil, Paraguay and Uruguay) and the water resources of the La Plata basin are
essential for their economic development.. The main tributaries are the rivers Parana,
Paraguay and Uruguay.
Some of the main developments having environmental impacts that influence
the basin have been the following: (i) developments of many hydro-power reservoirs in
the Upper Parana River, in Brazil, from 1960-1990; (ii) deforestation in the Parana,
Uruguay and Paraguay
basins from 1950-1990; (iii) introduction of intensive
agricultural practice after 1970; (iv) urban developments
with
change
to flood
regimes; (v) navigation and conservation of Upper Paraguay River.
Since 1970, flow increases have been observed which may have been caused by
changes in vegetation cover, or by climate variation. The changes raised important
issues both for water resource development and environmental conservation.
This paper discusses the water management and environmental issues, taking
account of climate patterns and the development of the five countries sharing the basin.
INTRODUCTION
The la Plata River flows through five countries in South America: Argentina,
Bolivia, Brazil, Paraguay and Uruguay.
Throughout almost its entire course, it is
known as the River Paraná, and only becomes the la Plata River below its junction with
the River Uruguay. This basin of the la Plata is one of the largest in the world and the
region has the highest level of development in South America.
Economical developments have had important consequences for the environment,
the main issues being the following:
(i)
deforestation and intensive agriculture in the Paraguay, Paraná and
Uruguay basins: the agriculture expansion since the 60´s, mainly in Brazil,
has left some areas with only 5% of natural cover;
(ii)
developments of many hydropower in the Paraná River: most of the
hydropower of Brazil, and indeed of South America, comes from this
basin. Dams have modified rivers behaviour and environmental conditions
both upstream and downstream;
(iii)
climate variations: the hydrological records show increases in both
rainfall and runoff after 1970. The increases have had important impacts in
areas such as the Pantanal, in the basin of the River Paraguay, and in the
basin of the River Paraná;
(iv)
environmental degradation in main cities: the population in the
region is highly concentrated in urban areas. The Brazilian State of São
Paulo with 36 million people, has 92% of its population in cities. Urban
development has had severe environmental impacts;
(v)
proposed improvements to navigation in the Upper Paraguay will
2
have grave consequences for the Pantanal wetland;
(vi)
flood hazard in the main rivers: floods of the main rivers have
resulted in extensive damage, particulary since the 80´s.
This paper presents some of the main characteristics of La Plata River Basin and
discusses some of the main issues related to development and environment
conservation.
THE LA PLATA RIVER
Main characteristics
In terms of area, the drainage basin of La Plata river is second only to
that of the Amazon in South America, and it is the fifth largest in the world. As
shown in Figure 1, its area of 3.1 million km 2 is shared among five countries
(Argentina, Bolivia, Brazil, Paraguay and Uruguay), through the principal subbasins of the Rivers Paraná, Paraguay and Uruguay.
The la Plata basin is important in different ways for the economy of each;
about 70% of the total GNP of the five countries combined is produced within
the basin, which is also inhabited by about 50% of their combined population.
Table 1, giving the distribution of drainage basin area by country and by subbasin, shows that almost half the total basin area is drained by the River Paraná,
and of this area 59%
lies within Brazil and 37.5% in Argentina. The River
Paraguay is the next largest sub-basin, making up a little over 35% of the La
Plata basin as a whole; of the area drained by the River Paraguay, about a third
lies within each of Paraguay and Brazil (32% and 34% respectively), 15% lying
in Argentina and
19% in Bolivia. The much smaller sub-basin of the River
Uruguay makes up about 12% of the total, of which about 43% lies in Brazil,
3
41% in Uruguay, and 16% in Argentina.
In the years 1967-1968, the five countries Argentina, Bolivia, Brazil, Paraguay
and Uruguay set up an Intergovernmental Committee which led to the signing of a
development
agreement
for
the
La
Plata basin.
Its aim
was
the rational
development of the basin in all its aspects. However, between the years 1969-1979
there were many diplomatic conflicts, mainly between Brazil and Argentina, because
upstream dam construction caused detrimental effects downstream. Early in the
decade
of
the
seventies,
Paraguay, Brazil and Argentina signed an agreement
concerning the operation of the hydropower plant at Itaipú, and this was followed
by various bi-national agreements such as those concerning Salto Hydropower in the
Uruguay Basin (agreement between Argentina and Uruguay); Yacereta Hydropower
on the River Parana (between Argentina and Paraguay) and the planned Garabi
Project (between Argentina and Brazil).
With the growth of Mercosul, which began as an agreement on commercial trade
among some South America countries and has now has been extended to others areas,
there are many projects in progress.
Some hydrologic aspects
The hydrological behaviour of the main rivers draining the la Plata basin is the
product of basin topography - itself a product of geology and climate - and
human activities within its boundaries. Natural conditions vary greatly from North to
South (the general direction of drainage) and from West to East. In the eastern half
of the basin, the basin boundary is clearly defined with mean altitude of 1000 m;
however the water divide can reach 1500 m in the extreme eastern part, but falls
to as low as 200 m in the South. On the western boundary, the Andean cordillera
4
reaches altitudes between 1000 and 4000 m, although there are stretches of this
western
basin
boundary, notably
in
the North-West
and
South-West, where
altitudes reach only to about 500 m and 300 m respectively.
Mean annual rainfall decreases both from north to south, and from east to west;
rainfall in the maritime uplands along the Brazilian coast reaches about 1800 mm,
but falls to 200 mm along the western boundary of the basin. Rainfall is greatest
5
in the upper parts of both the Paraguay and Parana River basins (Figure 2).
Table 2 presents mean annual flows at selected points within the three subbasins, together with the lengths of upstream sections and the areas drained. A
point of interest concerns the relative contributions to the La Plata flows of the
Rivers Paraná and Paraguay; where it joins the River Paraná, the Paraguay has a
mean annual discharge of
2 700 m
3
s
-1
, for a total drained area of
million km 2. At the outfall of the Paraná, on the other hand,
1.095
annual mean
discharge is 17 700 m 3 s - 1 , over six times that of the Paraguay, for a drained
area of 1 510 million km 2 . Thus the specific yield of the Paraguay is 2.47 m 3 per
1000 km
2
, compared with
11.72 m
3
per 1000 km
2
for the Paraná. The
difference (by a factor of 4.75) can partly be explained by high evaporation
losses in the wetlands lying in the upper reaches of the Paraguay, particularly
the Pantanal wetland described below.
Channel slope is very shallow for much of the River Paraguay. From
Cáceres to the sea is a distance of over 3 400 km, whilst the difference in
altitude between Cáceres and mean sea level is a few tens of metres. The poor
natural drainage of the region has created the Pantanal, one of the world's largest
wetlands with area approximately 140,000 km2 (Figure 1). Between the Pantanal
and Corrientes in Argentina, where the Paraguay joins the Paraná, the mean slope
is about 0.04 m km- 1 , falling to 0.01 m km- 1 in some parts. The slope of the
Pantanal itself is 0.25 m km- 1 in the east-west direction but only 0.01 m km- 1 in
the North-South direction. The time difference in the flood peak to the North and
South of the Pantanal is about four months, and the flow regime in the Paraguay
as a whole is strongly influenced by the Pantanal storage, one consequence being
that annual peak water levels in the Upper Paraguay are correlated between one
6
year and the next, regardless of rainfall conditions . We return to the hydrology of
the Pantanal later in this paper.
As is to be expected in such a large area, the principal matters of
hydrological concern vary considerably from sub-basin to sub-basin and from
upper reaches to lower reaches. In the Upper Paraná River, above the confluence
with the River Iguaçu, the principal matter of hydrological concern is the change
in land use from natural forest to arable cropping systems based on soybean
production. In the lower courses of
matters of
the Rivers Paraguay and Paraná, principal
hydrological concern are navigation, and flood control; in the recent
past, floods have caused significant loss of life and damage to property.
Table 1 Distribution of area between sub-basins and countries (OEA, 1969)
Sub-
Area
% of
Argentina
% of
Bolívia
% of
Brazil
% of
Paraguay
% of
Uruguay
%
basin
103km2
Prata
103km2
each
103km2
each
103km2
each
103km2
each
103km2
each
Basin
basin
basin
7
basin
basin
basin
Paraná
1 510
48.7
565
37.5
Paragua
1 095
35.3
165
15.0
Uruguay
365
11.8
60
16.4
Prata
3 100
100
920
29.7
205
18.7
890
59
55
3.5
370
33.9
355
32.4
155
42.5
1 415
45.7
y
205
6.6
410
13.2
150
41.1
150
4.8
Table 2 Some characteristics of the three principal sub-basins (Internave, 1990)
Main River
Paraná River
Junction of Paranaiba and
Grande
Junction with Paraguay
Mouth
Paraguay River
At Cáceres
Final section
Distance from upstream
km2
Basin
area
103km2
Mean anual
flow
m3 s -1
1200 (Paranaiba) and 1000
(Grande
2 540
3 780
376
4 370
975
1 510
11 800
17 700
420
2 620
1 600
33.8
1 095
365
345
2 700
5 500
Uruguay
WATER RESOURCE DEVELOPMENTS AND IMPACTS
The river basin is the region where water is available for planning water resources
use, yet demands usually occur according to a different geographic division, since, in
the past, rivers provided clearly defined boundaries. Thus, part of the basin is managed
by one state or country, and part by another which involve organizations from different
states or countries with different water use policies. The la Plata basin is an example of
this situation.
Basin planning involves definition and projection of water use in the basin. Often,
water resources planning is seen as the study of different alternatives for a specific
project, with one or more purposes, or for a water course reach. Planning for the whole
basin involves the integration of efforts to identify needs and define water use priorities
8
for multiple purposes.
Water use in the river basin has been dictated by national goals such as energy
production and water supply. The reservoirs have been designed according to specific
objectives which reflect economically tangible benefits. Consequently, areas around
headwaters reservoirs, which store water for irrigation, flood control or navigation
regulation reserve, do not enjoy the benefits or may makes things worse. Planning is
required to harmonize the multiple water uses and obtain tangible economics benefits,
as well benefits such as avoiding loss of human life and environmental conservation.
We now consider aspects of the main factors influencing the water resources of
the la Plata basin.
Hydropower
Hydropower power plants are usually sited in large basins and require significant
investment with eight years of planning. Their performance depends on two main
factors: hydraulic head and flow. Since these developments regulate large flows, large
volumes of storage are required, so that extensive areas must be flooded. Development
design in a large river usually makes use of regulating reservoirs and several others
with small volume and high hydraulic head. Due to the environment impact of large
flooding it becomes increasingly difficult to obtain approval for regulating reservoirs
which flood large areas. As a result, a cascade of small regulation reservoirs with high
heads
becomes likely,
with
important
consequences
for
the
downstream
environment.
Throughout the la Plata basin, the total capacity identified is about 92 000 MW;
53% of this total has been utilised or works are are in progress to exploit it. The
number of plants planned is significant, both at the domestic level of countries such as
9
Brazil, and as international projects.
The main areas with hydropower potential are on the upper Paraná River and on
the River Uruguay.
Brazil's energy is largely supplied by hydropower (93%), and
during 1965 to 1985 many dams were built for hydropower production (Figure 2) in the
Paraná River, which yields more than 50% of all Brazil's energy production. Today,
some hydropower plants are planned for in the Iguazu and other tributaries of the
Paraná (Piquirí and Ivaí). In the international reach, Corpus (downstream, of Itaipú) is
the dam which remains to be built but some difficulties await resolution, since although
the dam is to be built by Argentina and Paraguay, part of Brazilian territory will be
flooded.
The Uruguay River basin presents a large hydropower potential with a yield of
40,5 KW km
- 2
. The total energy available from the basin is 16500 MW, while the
total energy so far developed is 6680 MW.
Existing hydropower developments in the Uruguay River basin are on the Passo
Fundo River, also a tributary of the left bank in the international reach, and the Salto
Power Plant in the international reach between Argentina and Uruguay. The greatest
hydropower potential of the basin is found in the upper part, where several projects have
been planned .
Navigation
In the last century and in the beginning of the twentieth, the navigable waterway
of the la Plata tributaries provided the main channel for human settlement and
development. Today the principal navigable stretches (Figure 2) are:
10
(i)
On the Paraná-Paraguay: from the coast up to Cáceres in the Brazilian Mato
Grosso, a distance of 3,600 km. There is a major proposal to improve this
waterway to
increase
river depth, thus allowing more permanent traffic
through the countries having territory within the basin. This project will have
an important impact on the environment, to be discussed later in this paper;
(ii)
Tietê – Paraná: The Tietê river is one of the Paraná's main tributaries, flowing
through the highly developed region of the State of São Paulo which yields
about 36% of GNP of Brazil and has 36 million inhabitants. This waterway has
dams for navigation purposes, allowing transport by water from near the city
of São Paulo downstream to the River Paraná.
(iii)
River Uruguay: According to OEA (1985) the River Uruguay is navigable in
its lower reaches which form the border between Argentina and Uruguay up to
the dam of Salto Grande. Upstream from Salto Grande, the river is navigable
up to São Borja.
Agriculture and land use impacts
Until 1970 most of the agriculture in the State of Paraná and in a large part of
São Paulo State was devoted to coffee production under permanent cover, but after a
series of cold years, in the end of the 60s, coffee plantations were destroyed by
burning over a large area, with important economical losses. Subsequently, coffee
was substituted by annual crops such as corn and soya. This land use practice increases
the soil erosion in rural areas. At the end of
the 70s, some soil conservation
programmes were initiated, mainly in the Planalto towards the headwaters of the River
Paraguay, and in the upper reaches of the rivers Paraná and Uruguay (see Figure 1).
Soil erosion from the upper Paraguay is 4 t. yr-1.ha
11
-1
(Borges et al, 1996). In Rio
Grande do Sul the estimated erosion is from 16 to 32 t yr-1.ha -1 (Benetti et al 1989).
Another important area of erosion and river bed mobility is in the Bermejo River
which flows to the Paraguay River near Assuncion. Natural conditions of high bed river
slope, rainfall and soil have magnified the changes caused by human activities, and
cities had suffered from severe flooding.
Very little of the water in the upper Paraná is used for irrigation. In the Uruguay
there are some private developments for rice production. This irrigation occurs mainly
along the Ibicuí River, a tributary of the Uruguay. In this area there is some conflict
between the requirements for water supply and for irrigation during dry months. The
present demand is about 13 % of the mean flow.
Floods
Flooding is of major concern in the la Plata river basin. Most of the rivers have
long and wide flood plains which have been occupied by settlement or used for crop
production. The River Paraguay has a wide flood plain from its upstream reach in the
Pantanal down to its junction with the Paraná. Flood plains of the River Parana in
Brazil and its tributaries such as the Iguaçu,
contained many settlements which
experienced damage after 1970. Two major conurbations - São Paulo, with 16 million
inhabitants, and Curitiba with 2.5 million – occupy large areas of the flood plains of
the Rivers Tietê and Iguaçu, respectively. In the international reaches and in Argentina,
the River Paraná commonly extends over large areas during the flood season. These
areas are used for some agriculture and contain some important cities such as Santa Fé,
Corrientes and Rosário.
Over a considerable period (1950-1973), the annual floods were not extensive,
and this gave rise to the belief that settlements could be built on areas which
12
were subsequently shown to be at severe risk from flooding. The largest flood of
the century occurred along the rivers la Plata and Paraná in 1983, when for a year and
a half, the Paraná flood level was above street-level in parts of Santa Fé, Argentina,
although the city is protected by dikes. In União da Vitória on the River Iguaçu, the
cost of the flood amounted to US$ 78 million. In the State of Santa Catarina the
damages represented 8% of that State's Gross product for the year.
The steep gradient of the upper reach of the River Uruguay produces floods with
large and rapid changes in water level, whereas downstream in the international reaches
the plains can be under deep floods for long periods. Table 3 shows the number of
cities and population affected by the floods in some years.
Water Supply
Demands for water supply and irrigation can usually be met by small basins, in
which developments for water supply tend to be sited near to areas of demand.
However, water supply and irrigation may compete for available water, especially
during low flow periods, when demands increases.
Tabel 3 Population affected in Uruguay river basin during floods
Year
Loss of life Population affected
Number of
cities affected
1983
2
27,650
73
1984
1
12540
38
1985
0
4,680
5
1986
1
8,510
9
1987
1
7,200
6
1988
0
2,140
2
1989
0
3,510
3
1990
0
5,524
38
Source: Civil Defence: State of Rio Grande do Sul
Despite the fact that demand is not high relative to the available resource in the
basin, uneven temporal and spatial distribution of flow in the headwaters and water
quality degradation are of major concern in the la Plata basin, particularly in and
13
around the metropolitan areas of São Paulo and Curitiba. The city of São Paulo
requires about 60 m3 s - 1 of which 33 m3 s - 1 is imported from basins other than that
of the Tietê, which lacks water of a sufficiently high quality. Curitiba uses 7 m3 s - 1 for
water supply from the River Iguaçu but has some restrictions during floods because of
the contamination of the water and because water
catchment areas need
to
be
protected from urban development..
THE MAIN ENVIRONMENTAL ISSUES
The main basin developments and environmental impacts are the issues discussed
here. Due to the size of the basin, the choice of environmental issues discussed is
somewhat selective.
Climate variation, deforestation and agriculture practices.
Climate variation and its impacts have been a major issue for the la Plata basin
since the 70s, when the mean annual flow increased markedly in some river sections.
Table 4 shows flow increases from 19 to 46 %. Figure 3 shows the flood level
variation of the River Paraguay at Ladário and mean rainfall in the basin, both
standardised so as to be dimensionless. It can be seen that, after 1970 there is a large
increase in the level and a smaller increase in the rainfall.
Table 5 Mean annual discharges em m3 s - 1.
River Section
Before 1970
1970-1990
Parana River at Jupiá
5,852 (+)
6,969
(+)
R. Paranapanema at Rosana
1,057
1,545
R. Paraná at São José
6,900 (+)
8,520
(+)
R. Paraná at Guaira
8,620
11,560
(*)
R. Paraná r at Posadas
11,600
14,255
R. Paraná at Corrientes
15,265
19,510
+ series from 1930-1970; * series from 1901-1970
14
Increase
%
19,1
46,2
23,3
34,1
22,9
27,8
2
1,
1,
1,
4
1,
2
1
Rainfall
lLevel
0,8
0,6
Nondimensional P , H
0,4
0,2
0
190
191
192
193
194
195
196
197
198
199
2000
Year
Figure 3 Variation in annual water level at Ladário, (Upper Paraguay) and
rainfall (three year moving average) at Cuiabá, River Paraguay.
The main effects of increased flow and rainfall in the basin are the following:
(i)
soil erosion and sediment deposition in river channels, decreasing
the depth of soil available for agriculture;
(ii)
increase in river level and flood frequency;
(iii)
change in the river bed and the riparian environment ;
(iv)
decrease of reservoir volume capacity with sediments deposition;
(v)
increased production of energy from hydropower;
(vi)
changes in water quality due to resuspension of bed material
during flood conditions.
These increases in flow and rivers water level may be due to one or both of the
following causes:
15
(i) rainfall increase after 1970:Anderson et al (1993) using data from the River Paraná
at Corrientes (both rainfall and runoff) concluded that the unusual heavy precipitation
of 1980s and early 1900s was the most important factor accounting for the flooding of
these years. In addition, they also concluded that
there is no consistent evidence,
statistical or otherwise, that changes in rainfall-runoff dynamics associated with land
use changes played an important role in recent flooding in the Basin. The same paper
says There is, however, some evidence that when precipitation is not extremely high,
some change has taken place that causes streamflow to be greater than otherwise would
have been expected.
The mean annual rainfall from 1901 to 1970 in the Paraná basin at Corrientes was
1364 mm and for the period 1971 to 1991, 1438 mm, 5.4 % greater. This increase was
very similar to that observed in the part of
Brazilian State of Paraná. In theory, if all
the Paraná basin lying within the
the additional rainfall were to generate
runoff, an increase of 6% in rainfall over a river basin with runoff coefficient of 17%
(the value for the Paraná), the increase in runoff would be 35%. However this is, of
course, an upper limit to the increase in runoff..
Barros et al (1995) analysed rainfall trends in Southern South America to the East
of the Andes, citing many other authors who had concluded that rainfall has increased
in some parts of the la Plata basin. In particular, Castaned and Barros (1994) found an
increase from 850 mm during the 1920s to 1150mm in the 1980s in the mean rainfall for
humid Pampa. They found strong positive trends since the 1960s over the subtropical
region to the East of the Andes.
(ii) deforestarion and soil use: Bosch and Hewlett (1982), Bruijnzeel (1995) and
Sahin and al (1996) reported many experiments in small basin showing the following:
16
(i) deforestation increases the mean flow; (ii) deforestation followed by annual crops
which uses machinery for soil preparation are those practices which shows higher
increase in the mean flow.
The development of agriculture and settlement resulted in much deforestation in
the upper Paraná, Paraguay and Uruguay basins in Brazil. Table 5 shows the changes in
original cover in the States of Paraná and São Paulo, both of which lie in the basin of
the River Paraná.
In the State of Rio Grande do Sul, Southern Brazil, which hasone third of its area
is in the Uruguay basin, the forest cover at the beginning of this century was about 40%
of the total area of the State; today, it is estimated at just 2.6%.
It can be seen that although deforestation started before 1970, most of the
increase in flow occurred after 1970; but an important change in agriculture practice
from coffee to soya production occurred in Brazil after 1970 as described above.
Soya is a high value crop and its cultivation spread over large areas. Figure 4 shows
an example of the change in vegetation cover from one area of the Paraná basin. Soya
is also an annual crop which requires machinery for soil preparation (Figure 5 shows
the increase in machinery sold in the region after 1970). Such changes commonly
increase mean flow as reported above; but the area is large and the extent of the
impact of such changes at the basin-wide scale is not yet clear.
Tabel 3 Deforestation evolution in the States of São Paulo and Paraná in Brazil and
eastern Paraguay
Year
< 1886
1886
Original cover
of São Paulo
State
%
81,8
70,5
Year
Original Cover
of Parana State
%
Year
Original
cover(*)
%
< 1890
1890
83,4
83,4
1945
1960
55
45
17
1907
1935
1952
1962
1973
58,0
26,2
18,2
13,7
8,3
1930
64,1
1937
58,7
1950
39,7
1965
23,9
1980
11,9
1990
5,2
(*) eastern Paraguay ( Anderson Jr. et al ,1993)
(i)
1970
1980
1990
35
25
15
As can be seen, there are more questions than answers, and much
more research is required in order to solve them. The main conclusion are
(i)
that flow has increased in the Upper Paraguay, Paraná and Uruguay
basins; (ii) rainfall and land-use changes have both contributed to
cause the flow increase, although there is not yet a clear answer as to
the relative magnitudes of the two contributory causes.
In the context of water resource development and environment protection, the
main question is: how permanent is the increase in flow? In terms of energy
production, a permanent increase in flow represents greater firm energy yield, but
possibly a shorter reservoir life due to sedimentation; in terms of navigation, water
depth is increased and the period during which the rivers is navigable may be
lengthened; in terms of the flood regime, flood damage may be greater. The
question is difficult, requiring more research before quantitative answers can be
given.
18
Figure 4 Soil use change in a sampled area in North of Paraná (Kroner, 1990)
Pantanal: climate variation, soil erosion and effects of channel works
The River Paraguay and its tributaries flow from an upstream area called the
Planalto, with altitude greater than 200 m, into the large wetland of the Pantanal,
with altitude less than 100 m. The Pantanal is one of the world’s largest wetlands,
with area about 124.000 km2.
The ecosystem of the Pantanal is very distinctive and its preservation is important
for Brazil. Nevertheless, current human activities and proposed developments constitute
a considerable threat to its existence. Although cattle production in the Pantanal is
widespread, there is increasing mineral development, whilst intensive soya production
in the Planalto has increased soil erosion and sediment deposition in the Pantanal. Most
important of all is the proposal to extend the navigable length of the Paraguay in
order to open up central
South America to trade and for the export of the
region’s agricultural production.
19
Figure 5 Number of equipments sold in Paraná ( Parchen and Bragagnolo, 1991)
The consequences of this development must be carefully studied if the Pantanal is
to avoid total destruction. In addition, to establish how development will influence the
region it is first necessary to understand its hydrology.
Flow in the Paraguay and its tributaries is drastically reduced where they enter
the Pantanal, since the abrupt decrease in the gradient leads to deposition of sediment
on the river bed. The reduced erosive power of the river thus causes a reduction in
the cross-section relative to that farther upstream. In periods of flood, downstream
sections of the Pantanal rivers have smaller conveyance than those upstream, resulting
in extensive overbank spillage to a broader channel, and the greater the flood the more
extensive is the overbank spillage. However the plain of the Pantanal contains a large
number of depressions which are filled during floods, forming a landscape of small
lakes which merge together as the water level rises and which retain water as the level
falls again in the main channel. A large part of the inflow from upstream is therefore
retained as storage in depressions having no direct link with the main channels of rivers
flowing across the Pantanal. The flow and sediment which are the basis of the
ecosystem are reduced by about
half..
20
Climate: changes over time have been marked. The mean annual flood level at
Ladário on the River Paraguay fluctuated around 4 m from 1900 to 1960. During 1960
to 1973 the annual flood level varied around 2 m. This reduction of flow energy and
consequent siltation diminished the channel cross-section relative to that of the
preceding period. The inhabitants of the region - especially cattle ranchers - began to
utilise areas which previously had been flooded for long periods. After 1973, however,
the mean annual flood level increased again to around 5 m. Areas which had been free
of flooding for several months of the year became almost permanently flooded. During
the 1960s, the flooded areas of the Pantanal extended to about 17.000 km2, but after
1973 the flooded area exceeded 50.000 km2, reaching a maximum of about 100.000
km2 in 1988 (Hamilton et al., 1995).
Soil erosion: In the Planalto, there has been a dramatic increase since the 1970s
in areas planted to annual crops, principally soya, which has resulted in significantly
increased soil erosion and sediment transport to the Pantanal. At the same time, the
short-term increases of annual rainfall in the upper part of the basin have caused soil
loss in the Planalto with deposition in some reaches and, in the Pantanal, greater
deposition of sediment and reduced channel conveyance.
Within the Pantanal itself, land-use changes have been largely restricted by the
hydrological regime, although some dykes have been constructed to control flooding.
However during floods, rivers may change course by cutting across meanders, and
where this results in a land-owner losing part of his property, he may seal off the
new course, so that the river will revert to its old one; where this occurs, there is a
consequent high mortality of fish and other aquatic life in the cut-off section.
Paraná- Paraguay waterway: This is a 3 600 km long waterway, from Nueva
Palmital, near the coast, to a point upstream of Cáceres in Brazil (Figure 6). To
21
improve transport, works are planned that will improve the river channel through the
entire waterway. The first project were presented by Intervave and the last one by
Hidroservice-Louis Berger-EIH. This last project presented the following alternative:
Figure 6 Paraná-Paraguay waterway
(i) from Santa Fé (Argentina) until Assunción (Paraguay) a channel 100 m wide
and with 3m deep;
(ii) from Assunción to Corumbá (Brazil), a channel 90 m wide and 2.6 m deep;
(iii) from Corumbá to Cáceres, many works are proposed that will result in at
least 1.5 to 1.8 m of water depth.
A major concern is the environmental impact on of these works on the
Pantanal wetland. The works will improve the river conveyance which may decrease
the flood area. The decrease in over-bank flow and flood-plain sedimentation will
tend to change the Pantanal from wetland to savannah, since the difference between
22
rainfall and potential evapotranspiration is negative. In a sequence of drought years,
the effect would be even more critical.
The main question relating to this issue are:
(i) Will the proposed works in the waterway modify flow conditions so as to
reduce flow volume to the flood plain, and if so, by how much will it be
reduced?
(ii) What would be the effect on the flood-plain environment of a sequence
of drought years ?
More research
based in field data is required before these questions can be
satisfactorily answered.
CONCLUSION
The basin of the la Plata is one of the world's largest, and its 100 million people
are responsible for a major part of the economy of South America. Countries sharing
the basin are receiving high investment and have a high index of economic growth,
with consequent pressures on the environment.
This paper lists the main environmental issues related to water resources and
discussed the following issues:
Climate variation, agriculture practices and flow increase. Increased runoff
after the 1970s may be due to rainfall and agriculture practices, or both. If
rainfall has been the principal cause of the increased runoff, the increase
may not be permanent and
1960s;
but if
changes in
river behaviour may revert to conditions of the
land-use
and
agricultural
practices
have
contribued to the increased runoff, it is possible that higher flows are more
23
permanent. If they
are, this
has important economic and environmental
consequences for strategies for sustainable development, which will need to
take into account: (i) increases in water depth, and in duration of navigable
conditions,
in
the
watercourses; (ii) increased
energy production; (iii)
possible decrease of reservoir capacity from sedimentation; (iii) more extreme
flood conditions. It will become more necessary than ever to: (i) improve
agriculture practices for water and soil conservation; (ii) develop
non-
structural measures for flood control in critical areas, such as: flood zoning,
real time flood forecasting and insurance.
Pantanal wetlands conservation: The Pantanal is a major wetland environment
of South America which has been greatly influence in the last thirty years by
climate variation and changes in land-use and agriculture practices. In addition, a
waterway has been proposed for the la Plata basin countries which could greatly
affect the wetland system.
In broad terms, increasing the channel transport
capacity by dredging, as planned by the waterway improvements, would reduce
the volume of water retained in the Pantanal wetland, possibly changing it from
wetland to savannah, but sound quantitative evaluation is necessary to establish
how and where the changes would occur before any decision of investment is
taken.
Sustainable development of this region must be achieved without damaging its
natural behaviour and an important conservation planning programme is required
to reconcile the conflicting requirements of conservation and economic
development.
24
References
ANDERSON, R. J. ; RIBEIRO, N. F.; DIAZ, H.F.. 1993. An analysis of Flooding in the
Parana/Parguay River Basin. The World Bank Latin America Technical
Department.
BOSCH, J.M.; HEWLETT, J.D., 1982. A review of catchment experiments to
determine the effect of vegetation changes on water yield and evapotranspiration.
Journal of Hydrology 55: 2-23
BRUIJNZEEL, L. A , 1990. Hydrology of Moist Tropical Forests and Effects of
Conversion: A State of Knowledge review. IHP. IAHS. UNESCO. 224 p.
BARROS, V.; CASTANEDA, M. E.; DOYLE, M. 1995. Recent precipitation Trends in
Southern South America to the East of the Andes: and indication of a mode of
climatic variability. Proceendings of Latin America Workshop on Greenhouse gas
emission of Energy sector and their Impacts COPPE/ UFRJ, Rio de Janeiro.
HAMILTON, S.K.; SIPPEL, S.J.; MELACK, J.M.1995. Innundations Patterns in the
Pantanal wetland of South America determined from passive macrowave Remote
Sensing. Hydrobiologie, January
KRONER, 1990 A erosão do dolo de 1952 a 1985 e seu controle no Paraná. Londrina
IAPAR Boletim técnico n. 30 53 º
OEA, 1969.
Bacia do Prata. Estudo para sua Planificação e Desenvolvimento.
Organização dos Estados Americanos.
PARCHEN, C. A. P. ; BRAGAGNOLO, N., 1991. Erosão e Conservação de Solos no
Paraná. Emater – PR.
SAHIN, M. J.; HALL, M.J., 1996 The effects of afforestation and deforestation on
water yields Journal of Hydrology 1178 p293-309
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