| THE PARAGUAYAN CHACO AND ITS HYDROLOGY
Victor M. Ponce and Juan P. Nogués
The word Chaco originates from the Quichua word "chacú,"
which means "place of hunting." The Chaco was a well
known place by the Incas; they would migrate for a few
months each year to the Chaco to hunt and to give renditions
to their gods. Later, during the colonial period, this region
was hardly noticed and there was no land exploitation. Today,
80% of the economy is sustained by cattle ranching and another
10% by agriculture.
This remote land in South America, whose limits remain controversial,
includes parts of Argentina, Bolivia, Brazil and Paraguay.
The Chaco has an area of about 1,500,000 km². It is divided
into two clearly identifiable ecoregions: the humid Chaco and
the semiarid or dry Chaco (Fig. 1). The climate is extreme,
ranging from -7oC to
Both of these ecoregions are clearly distinguishable in the
The western portion of Paraguay, limited to the east by the Paraguay River,
also known as the Paraguayan Chaco or Chaco Boreal, has approximately
246,925 km², which represents 25% of the South American Chaco and
60% of Paraguay. The population density is 0.04 inhabitants
per square kilometer, and it is home to only 2.5% of the Paraguayan population.
Three departments, Alto Paraguay, Boquerón
and Presidente Hayes, compose the Paraguayan Chaco. Of these three,
Boquerón is the only one that has not shown a clear increase
in population in the past 10 years;
it is also the department with the largest indigenous population.
The Chaco has a general gradient sloping east;
it is a great
alluvial plain, which has been formed with aeolian, fluvial
and marine deposits providing deep soils, free of rocks.
rains are generally concentrated in the southern zone. Floods
are common in this region, which is characterized by its
palm trees and sugarcane fields, and where it rains approximately 1300
to 1500 mm per year.
In the southern Chaco, the flat wetlands provide
a habitat for wet vegetation, such as marshes and tall weeds.
In the northern Chaco,
the tectonic faults and the arid climate provide a habitat
for savannas. Near the Bolivian border, a very undulated topography
is found, with sand dunes of low elevation; this could be considered
the start of the Andes mountain range. Near the Bolivian border,
the Cerro León chain has elevations of about 125 m. The northern
region has an average of 500 mm of rain per year. Northern soils
have been barely used, mainly because of the long distance to commercial
markets, its arid characteristics and the general lack of knowledge about
The Paraguayan Chaco comprises an area larger than the state
of Oregon, Great Britain and eight Latin American countries.
Nonetheless, it is still hardly populated, and studies
for sustainable development of the land have yet
to be accomplished.
Although the Paraguayan Chaco is wholly contained within the
Paraguay River basin, it is necessary to analyze other hydrologic
aspects to understand the region's peculiarities. Mainly, the
two most important rivers are: (1) the Paraguay, which contains
several slow meandering tributaries along the way; and (2) the
Pilcomayo, which feeds Estero Patiño, one of the better
preserved wetlands in the Chaco. Downstream of the
wetlands, the Pilcomayo drains into the Paraguay River close
to the city of Asunción. Continuing downstream, the Paraguay
River eventually joins the Paraná, to finally end at the
Río de La Plata basin, and into the Atlantic Ocean. In a hydrologic
sense, the Paraguayan Chaco is conditioned by three elements:
local precipitation, the hydraulics of the Pilcomayo River, and the Paraguay
River and its tributaries (Fig.
|Fig. 2 Limits of the two ecosystems
found in the Chaco.
The Paraguay river originates
upstream of the city of Cáceres, Brazil, north of the
Brazilian Pantanal of Mato Grosso. The basin has approximately 1,000,000 km²
from its headwaters to the southwest corner of Paraguay, where it drains
into the Paraná river. The average discharge is 2500-3500
m³/s with the maximum being 7000 m³/s. Records indicate a low
saline concentration, a pH of 7, alkalinity of 25 mg/L, hardness
of 26 mg/L and suspended solids of 12 mg/L. As
the river heads towards the Paraná the salinity increases.
All the water that drains from the Brazilian Pantanal ends
up in the Paraguay River and the Paraguayan Pantanal. The widest
portion of the basin is at Corumbá, with approximately
300 km. When the river reaches Porto Esperança, south
of Corumbá, it makes an abrupt turn heading southwest
as it enters the Paraguayan Chaco (Fig.
3). Once the river is in the Chaco territory, it regains
its natural direction (north-south). From there, the Paraguayan
Pantanal extends on both sides until it reaches the Apa River
The same vegetation as in the Brazilian Pantanal can be seen
along the Paraguay River for 250 km with varying width, from
40 to 10 km (Fig. 5).
The flood plain occupies an area of 400,000 ha, which in times
of high floods can reach about 600,000 ha. This area has the
highest medium temperature of the Chaco, about 25 oC
year round and an average rainfall of 1100 mm. Upstream, where
the Chaco and the Brazilian Pantanal meet in Bahía Negra,
the channel velocities are very slow; therefore, when high floods
occur, they take three or more months to reach the southern
Chaco. The southern Chaco receives Pantanal floodwaters in the
months of October and November. The waters of the Paraguay River
in the southern Chaco from the Apa river to its end in Asunción
have higher velocities; therefore, sand beaches, sand banks
and islands on the margin start to appear, as well as river
There are numerous tributaries of the Paraguay
River extending from south of the Pilcomayo River to the
northern Chaco Pantanal. All of these watersheds are within the
territory of the southern Chaco. Most of the Paraguay River
tributaries are not navigable, but are important because of the
irrigation water they provide. Among the most important are:
Negro, Siete Puntas, Confuso, Verde, Montelindo and Aguaray-Guazú
on the right margin; and the Apa, Aquidaban,
Ypané, Jejuí and Tebicuary (which are located
on the eastern region of Paraguay) on the left margin.
The flows and
reaches of the rivers are shown in Table 1.
|Table 1. Main tributaries
of the Paraguay River.
Average Flow (m³/s)
The central Chaco's drainage is made of paleochannels
and intermittent gorges which are dry most of the year, and
transport overflows in the rainy months of the summer to the southern
tributaries of the Paraguay River. The region has an average
slope of 0.0003, draining towards the east.
Upstream, on the Paraguay River, there is a complex
drainage web made of the Yacaré, Pacú,
San Juan, Mosquito and Negro rivers. All of these are perennial,
but have a very short length and are mainly dependent on
the water levels of the Paraguay River. During the summer months,
which coincides with low water levels in the Paraguay River
and with rain in the surrounding areas, the rivers flow eastward
and drain into the Paraguay River. In the months of high water
levels (May through August), the backwater reaches
50 to 80 km westward.
The Pilcomayo river originates at 4000 m above sea level in the
Bolivian andes, north of the city of Potosí. After flowing
for about 500 km in the Bolivian valleys, plateaus and canyons,
it enters the South American Chaco through the city of Villamontes,
where the gradient is reduced from 0.045 to 0.033. The upper
basin, located in the Andes, has a surface area of about 80,000
km², while the lower basin, properly in the Chaco region,
has an area of about 180,000 km². From Ybybobo in Bolivia,
the river wanders through strong meanders; in the rainy season
the flow increases and so does the velocity. Substantial erosion
is seen in its concave banks, due to the meandering action.
Once at the Horqueta Port (42 km SW of General Díaz), the river
starts to digress, transforming itself from a clearly defined
channel into a series of wetlands and marshes, such as the Patiño
and Tinfunke marshes. The water feeds some temporary basins,
infiltrates the soil, or evaporates. At Salto Palmar, the river
regains its original course and direction, with a smaller flow
and sediment load, but more salinity. This last extension of
the river runs for 240 km as it drains into the Paraguay river
south of the city of Asunción.
The Pilcomayo is the longest
tributary of the Paraguay River in the Chaco. The rivers Confuso (Fig. 6),
Negro, Aguaray-Guazú, Verde, Siete Puntas and Montelindo
compose the Pilcomayo delta. Of these, the first three receive
an important amount of overflow from the Pilcomayo, plus draining
the water of their respective watersheds. The salt level in
the water increases from the Confuso upward, as the rivers part
from the Pilcomayo to the south, which carries sweet water.
Another peculiarity of the Pilcomayo is that along its course
in the Paraguayan Chaco, it does not receive water from a single
tributary (Fig. 7).
The hydrologic system of the Pilcomayo River in the Chaco consists
of the following components: (1) The Upper Pilcomayo, which encounters
its hindrance 70 km south of the Paraguay-Bolivia border; (2)
gorges that are formed at that point and transport the water
to (3) the marshes of Tinfunké and Patiño; (4)
the rivers Montelindo, He-é, Siete Puntas, Verde, Confuso
and lower Pilcomayo, which are born out of the marshes and finally
drain into the (5) Paraguay river.
|Fig. 8 Detail of the Pilcomayo River.
The two characteristics that stand out the most in the hydrologic
and fluvial phase of the Pilcomayo are: (1) the extreme variability
in flow discharge, ranging from 3 to 3000 m³/s, in the
periods of drought and floods, respectively. Secondly, there
is the pronounced erosion process that occurs in the upper basin,
with an average sediment load of 98 million tons per year. The
Pilcomayo deposits about 50,000 m³ of sediments per year,
with water densities ranging from 50 to 100 kg/m³
9). The cause of this phenomenon is due to the fact that in
the upper basin of the Pilcomayo meanders through the Andes,
where outcrops of erosional rocks and an intense rainfall regime
during the summer months produce great quantities of sediment.
Records dating back to 1944 show the
retreat of the Pilcomayo, due to sediment deposition
in the downstream marshes. To this date,
the river has lost 300 km in
length, and the end of its main channel is now located only
70 km from the Bolivia-Paraguay border. The Patiño marsh,
which is located 250 km south of this point, is receiving a
small amount of flow, harming the local fauna
In an attempt to lessen the effects of this retreat,
the governments of Paraguay and Argentina have undertaken since 1991
a series of actions to slow this natural phenomenon. These
actions led to an irregular distribution of channels along the
sides of the Pilcomayo. From 1991 through 1994, most of
the low and medium flows were diverted to Argentina. From 1995
to 1996, these levels were diverted to Paraguayan territory. Since
1997, this flows have again been diverted into Argentinian territory.
Another important characteristic of these
rivers is that their basins are not unequiivocally defined, due to
the relatively uniform morphology of the terrain. This phenomenon causes
a transversal action in the watersheds, increasing them or changing them,
in many cases due to anthropogenic actions (i.e., construction
of channels, roads, dams, etc.). Therefore, the effects of the
increase of water level in the Pilcomayo are variable and unpredictable.
Under normal conditions, the flow is transported to the lower
Pilcomayo and the Montelindo rivers; under extreme conditions, flows
can reach as far as the Verde river.
The extreme variability
of flows is attributed to the lack of control in the upper basin.
Currently, the high waters are diverted to the Argentinian side
of the Chaco and substantial harm is being done to downstream
marshes, which sustain a very delicate and intricate ecosystem.
The marshes of Patiño and Tinfunké are of great
importance, not only to the rivers that are born there but also
because they constitute a very important feeding point for the
fisheries. Upstream of the marshes, the penetration of sunlight decreases
due to the heavy concentration of solids, such that microorganisms
that sustain fishes are not able to proliferate.
The Pilcomayo presents two distinct hydrologic moments: (1) the
period of low flows, extending from May to October
of each year, which coincides with winter and small rain; during
these periods, flows are as low as 3 to 10 m³/s; (2) the period of high flows,
during the summer,
when high flows are of the order of 1000 to 3000 m³/s;
these high flows give rise to the wetlands and the excess flows
that the smaller rivers receive.
There is no set cross-sectional area for the channel bottom,
because the bed is always moving; this, added to the fact that
the overflow is irregular during floods, makes it hard to develop
a trustworthy hydrometric analysis. Therefore, the Pilcomayo basin is one in which all the
parameters are presented in an extreme fashion, constituting
a challenge to modern technology. Such challenges encompass
the analysis and comprehension of the phenomenon, the design
and execution of human interventions, as well as a search for
a new institutional and political framework based on the river's
In this natural region of 113,400 km², the terrain is
dry, with thorny bushes being its main feature. It has an ondulating
characteristic, an indication of the earlier rivers that used
to flow through the region. The flora and fauna seem to have
reached equilibrium, but the biological aspects are still dependent
on the hydrologic cycle. With small exceptions, there is no
water available for 180 days each year. Periods of humidity
do not last longer than 90 days straight; 80% of the rain comes
in the summer months, producing a monsoon-like environment.
The sandy texture and the dryness make soils very porous and
permeable. Nonetheless, the type of monsoonic rains that occur
during the summer saturate the soils too fast for the entirety
of the water to be absorbed; therefore, ending as runoff into
an ephemeral river. As a general rule, in the central region
of the Chaco there is a drought every 7th year, with rains of
600 mm/year. As one moves to the north, the annual average reduces
to about 400-500 mm and the evapotranspiration is of the order
of 2000 mm per year.
In the north, subterranean waters are the main source of supply and
travel in a southeastern direction.
These waters are formed
through a complex process of infiltration that begins as the
waters are washed in the saline deposits of the Andean
mountain range. If sweet waters, especially from the Pilcomayo
floods, did not superpose these aquifers, there would be a risk
of generalized salinization that could later lead to desertification.
On the northwest, there are fresh-water artesian aquifers (20
to 30 m) along the Pilcomayo and near the Bolivian border. Moving
towards the center and the east, the soils become less sandy
and it is thought that due to a geologic fault, the grade changes
and the aquifers are at greater depth (100-200 m). Aquifers become
very saline and rich in minerals (5000 mg/L), rendering them useless
for irrigation purposes.
The most important river in the area is the Timane (located
near Pablo Lagerenza), which is said to be very curious because
it lacks a beginning or an end. In fact, during times of high
flows, almost all the water accumulates in lakes; the two most
important lakes being Palmar de las Islas and Trinidad. It is an intermittent
fresh-water channel with an average flow of 134 m³/s. The basin
receives an average of 600 mm per year of rainfall; the drainage
area is 56,350 km² and the evapotranspiration is 718 mm. The Timane
receives flood waters from the Parapití and the Yzozog
river of Bolivia, producing high floods without any
precipitation. Of the northern lakes, few studies have been done
about the capacity, storage and quality, and most are drying
out and are only of geographic use.
The area around Pablo Lagerenza (Northern Chaco) receives
500 mm of rain per year, while in Fortin Ravelo
(Bolivia), about 50 km from Lagerenza, the rainfall amount is 1064 mm per
year. This shows the extreme variability of water supply
in the northern Chaco and its surrounding areas.
|Though the South American Chaco is noted by its dryness and
arid conditions, in the southern Paraguayan Chaco, there exists a
system of wetlands originating from three hydrologic elements:
(1) the overflow waters from the Paraguay river, (2) floods from
the Pilcomayo river, and (3) rainfall activity in this area. The
soils are of clayey texture, inhibiting infiltration; therefore,
during the months of heavy rain most of the area is flooded. On
the average, the region is flooded four months out of the year, cutting
road communication in some areas (Fig.
10). In the transition zone between the arid and the humid
Chaco, the topography is a little more elevated, causing high
runoffs and lesser floods.
The floods occur in the months of summer,
where the stage on the Paraguay is low; during winter, when there
are high stages on the Paraguay, the water is diverted inwards
in some cases, but not to the extent to cause floods. Inland-diverted
waters in the south can reach from 80 to 150 km upstream.
The aquifers in the southern region are even more saline,
sometimes so much that they cannot be used as drinking water.
Nonetheless, the fields get enough rain water, 1300 to 1500 mm
per year, so the cattle raising industry does not suffer from
this condition. The rains that cause floods during the summer
months are not drained by adequate mechanisms; therefore, most
of the water is not used in an efficient way. This lack of potable
water and the resources to drain them efficiently, causes some
problems. Water harvesting by building large water tanks
of clay or by collecting rainwater from the roofs is the main
way to collect potable water.
Fig. 11 Palm field in the lower Chaco.
In the Chaco, the wetlands are extended horizontally, and depth
can range from a few centimeters to 1 or 2 m, with emerging
vegetation. Wetlands can further be divided into two types: (1)
marshes, which have water permanently, and (2) washes that contain
water momentarily due to floods or rain.