River–groundwater interactions in the Brazilian Pantanal.

The case of the Cuiaba River

Pierre Girard*, Carolina J. da Silva, Mara Abdo

Projeto Ecologia Pantanal, Instituto de Biociencias, Universidade Federal de Mato Grosso, Av. Fernando Correia da Costa s/n,

Bairro Coxipo, 78060-900 Cuiaba, MT, Brazil

Received 19 February 2002; accepted 23 June 2003

Abstract

The Pantanal is a vast evaporation plain and sediment accumulation surface that floods annually. It is located in the Upper

Paraguay River Basin, a major source of floodwaters to the Pantanal. The recent construction of a large dam in the upper reach

of the Cuiaba River raises questions: What will be the dam influence on the flood area and duration? What will be the

consequence for groundwater replenishment and permanence of flow in the floodplain channels during the dry period? This

study of the Cuiaba River, within the Pantanal, describes water flow between the river channel and its adjacent floodplain, as

well as relations between the surface water and groundwater near the river. Flooding of the plain adjacent to the Cuiaba River

critically depends on the river stage and proceeds through a complex hydrographic network. No free water table was

encountered; groundwater was confined below clay–silt layers. Two groundwater bodies were distinguished based on their

piezometric behavior. In both cases the river stage variations appeared to control the piezometric heads and the flood was the

main recharge source. The groundwater moved from the river towards the floodplain where it appeared to sustain channel flow

and to maintain soil humidity in depressed areas during the dry period.

q 2003 Published by Elsevier B.V.

Keywords: Pantanal; Flood pulse; Groundwater; Recharge; Ecological stability

1. Introduction

The Pantanal wetland is a vast evaporation plain

and sediment accumulation surface (Junk et al.,

2003) occupying an immense sedimentary

depression. It is located south of the Amazon Basin

and east of the Andes, in the Upper Paraguay River

Basin. Annually, the Upper Paraguay River and

its tributaries flood an area of about 140,000 km2

(Junk, 2000). This land is still in a rather pristine

state and its ecological integrity is tightly linked to

its hydrology (Da Silva, 2000). Recently, however,

environmental disturbances such as deforestation due

to the expansion of the agro-industry and the

consequent erosion and sedimentation have

increased. A new threat, the disruption of the

fundamental seasonal flood pulse by engineering

works such as waterways and large dams, is also

emerging. One of these dams, which started operat-

ing at the end of 1999, is now regulating the flux of

the Cuiaba River, one of the main sources of flood

0022-1694/$ - see front matter q 2003 Published by Elsevier B.V.

doi:10.1016/S0022-1694(03)00235-X

Journal of Hydrology 283 (2003) 57–66

www.elsevier.com/locate/jhydrol

* Corresponding author. Fax: þ55-65-615-8264.

E-mail address: pgirard@terra.com.br (P. Girard).

waters to the Pantanal—the largest river after the

Paraguay River itself.

In spite of the environmental impact study that was

performed before the dam construction, the body of

knowledge concerning the flood hydrology of the

Cuiaba River in the Pantanal is still small. The most

important study was realized by the Conservation

Plan for the Upper Paraguay Basin Project (PCBAP,

1997), whereby the available hydrological data were

inventoried and summarized. Hamilton et al. (1996)

also studied the seasonality of Pantanal rivers in

relation to flood extent. However there is still no

description of how the flooding proceeds and how the

river and groundwater interact.

The aims of this paper are to contribute to the

hydrological knowledge of a reach of the Cuiaba

River located within the Pantanal, to describe water

flow between the river main channel and its adjacent

flood plain, and relations between the surface water

and the groundwater near the river.

2. Study area

The study area is located near a colonial bird

nesting area, locally known as Ninhal Corutuba

(Fig. 1). This Ninhal is on the margin of the Cuiaba

River in the northern portion of the Pantanal. In the

next paragraphs, the Pantanal, the Cuiaba River at

proximity of the study site, and the Ninhal are briefly

described.

There is an emerging body of literature describing

the Pantanal (Adamoli, 1981; Alvarenga et al., 1984;

Da Silva, 2000; Da Silva et al., 2001; Hamilton et al.,

1996; Junk et al., 2003; PCBAP, 1997; RADAM-

BRASIL, 1982) from which the following description

was assembled. The Pantanal is located along the

course of the Upper Paraguay River, an important

tributary of the Parana, one of earth’s largest rivers.

The Upper Paraguay drains an area close to half a

million square kilometers of which two thirds are in

Brazil, in the states of Mato Grosso and Mato Grosso

do Sul. According to topographical elevation this

basin can be subdivided in three physiographic units.

First, the ‘Planalto’ or ‘Plateau’, 250–750 m a.s.l, is

the headwater region. A flat undulating plain con-

stitutes most of the Plateau and it is covered by a more

or less open savanna—locally called ‘cerrado’—that

is now extensively used for agriculture and cattle

ranching. Next, there is the ‘Depression’ at altitudes

ranging from 180 to 250 m. This is a small region with

generally steep slopes that is covered by a dense forest

locally called ‘cerradao’. The last unit is the Pantanal,

from less than 100 to 180 m altitude; it is about half

the size of the Plateau. It is a low relief plain with

hydraulic gradient not exceeding 15 cm/km. Many

large rivers, such as the Paraguay, the Cuiaba, the Sao

Lourenco, the Piquiri, the Taquari, and the Negro

cross this vast plain. This complex hydrographic

network in conjunction with diverse soil types gives

rise to a variety of landscapes within the Pantanal.

These large sub-units are subjected to different

hydrological conditions, and different plant commu-

nities characterize them.

The climate of the Pantanal is marked by a

pronounced dry season from May to September and

a rainy season from October to April. Mean monthly

temperature near Cuiaba City varies between 27.4 8C

in December and 21.4 8C in July. Short-term

ingressions of polar air masses in winter may cause

the temperature to drop as far as 0 8C. Annual rainfall

decreases from 1250 mm in the northern Pantanal to

1089 mm in the south. Evapotranspiration, ranging

from 1100 to 1300 mm yearly (Hamilton et al., 1997;

Ponce, 1995), surpasses precipitation during at least 6

months per year. Mean monthly air humidity varies in

the northern Pantanal from 84% during the rainy

season to below 60% at the end of the dry season,

when the floodplain is dry (Tarifa, 1986).

The annual flooding is caused by the sharp gradient

contrast between the Depression and the plain

(Carvalho, 1986). Some geomorphological constric-

tions, namely rock outcrops, along the Paraguay River

reduce even more the ability of the river to drain away

floodwaters. In addition, local rainfall on the flood-

plains drains slowly, enhancing flooding. The Panta-

nal is essentially a huge, gently sloped basin that

receives runoff from an upland watershed—the

Plateau—twice its size and slowly releases the flood

pulse of those waters through a single, downstream

channel, the Paraguay River (Ponce, 1995).

The annual flood pulse is mono-modal and presents

temporal and spatial variations. Along the main river

channels the annual pulse is sharp and well defined,

driven mainly by river overflow. Farther from

important channels, the flood pulse is more attenuated

P. Girard et al. / Journal of Hydrology 283 (2003) 57–6658

Fig. 1. The Ninhal Corutuba study site. The top RADARSAT image shows the study area during the rising waters (January 1999). The

bottom view is during the low waters (October 1999). The insert in top view shows the location of the Brazilian Pantanal, the study site

and Cuiaba City. The insert in the bottom view is a schematic representation of the study site with the location of staff gages (g) and

piezometers (P4–P16).

P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 59

(Penha et al., 1999). As the surface slope along the

Paraguay is about 3–5 cm/km, and because of the

rock outcrops mentioned earlier, the flood pulse

moves slowly southwards and there is a lag of 4–6

months between the flood peaks in the north and in the

south. Most of the water enters the northern Pantanal,

as the three major contributors are the Paraguay, the

Sao Lourenco and the Cuiaba Rivers. The flood pulse

maintains the biodiversity and health of the Pantanal

ecosystem (Da Silva and Esteves, 1993; Espindola

et al., 1996; Penha et al., 1998, 1999; Resende et al.,

1996; Strussman, 1991).

The vast fluviolacustrine plain of the Cuiaba River

consists of actual alluvium that forms fluvial islands,

marginal levees, and bars. Grain size varies from

sands to clays and most abundant are sandy silt and

clay–silt beds that are often intercalated.

The Ninhal Corutuba is located on the left margin

of the Cuiaba River (Fig. 1). There, the Cuiaba

Channel splits in two and its smaller arm is called

‘Jacorutubinha’. The Ninhal is located where the

Jacorutubinha meets the Cuiaba main Channel

(16828018700S and 56807053600W). In this region the

Cuiaba River flows over its own fluviolacustrine

sediments. Along the river course there is a series of

small lakes seasonally linked to the Cuiaba and

abandoned meanders. There is also a high density of

tie channels in which the water may flow both ways

and other channel types that present a parallel

drainage pattern.

3. Material and methods

3.1. Institutional data acquisition

The Cuiaba River stages were acquired from the

ANEEL (Brazil’s National Agency for Electrical

Energy—www.aneel.gov.br). The complete daily

readings from the Cuiaba, Barao de Melgaco and

Porto Cercado region, from the beginning of 1998

until end of March 1999, were obtained.

3.2. Fluviometric and piezometric data

at the study site

There were two study locations, one in the Ninhal

and one in a nearby riparian forest lacking a bird

nesting area. At both sites staff gages were installed.

They consisted of four wood posts standing about 1 m

above the ground level installed in a stairway fashion

from the top of the left bank levee. The water levels

were measured from the top of the post with a

conventional meter. The accuracy is ^0.001 m.

Standpipe piezometers were installed in both sites.

They consisted of PVC tubes ðf ¼ 32 mmÞ that were

manually slotted in the lower 0.5 m. The slotted

section was covered with a fine mosquito screen.

These instruments were inserted in manually bored

holes ðf ¼ 60 mmÞ: The slotted section was installed

within the aquifer sand formation. The slotted section

was coated with local clean quartz sand and covered

with a concrete plug to prevent floodwater infiltration.

Static level within the piezometer was measured from

the instrument top with a sound indicator. The

accuracy is ^0.005 m. Five piezometers were

installed at the Ninhal (P4–P8) and two at the forest

(P12 and P16) site. Approximate locations of all

instruments are given in Fig. 1.

At each site, the relative elevations of the

instrument tops were determined. The highest instru-

ment at each site (the staff gage on the top of the

Jacorutubinha levee) was arbitrarily assigned an

elevation of 100 m. A transparent tube full of water

was then used to find the elevation of the other

instruments.

Readings from these instruments were obtained at

least once a month from November 1998 to December

1999.

4. Results and discussion

4.1. Hydrological framework

There are no historical fluviometric records in the

Jacorutubinha channel. However, the Cuiaba River

has several fluviometric stations. The longest record,

starting in 1933, is from the Cuiaba City station some

75-km upstream of the study site. The Cuiaba is a

strongly seasonal river and Fig. 2 shows the hydro-

graph of the historical mean monthly stages as well as

the variation of the historical mean monthly minimum

and maximum. The mean monthly stages for the year

1998 are also displayed showing that this was an

atypical year, as the river stages were unusually low.

P. Girard et al. / Journal of Hydrology 283 (2003) 57–6660

This was also the case in the Pantanal at the Barao de

Melgaco and Porto Cercado stations.

In the Ninhal, the river stages were recorded only

once a month, but daily stages available upstream

(Barao de Melgaco) and downstream (Porto Cercado)

on the Cuiaba River, provide a good indicator of the

hydrograph shape in the Ninhal. Compared to the

Barao de Melgaco record, the stage at Porto Cercado

was smoother during flood time (Fig. 3), due to the

fact that, unlike in Barao de Melgaco, the Cuiaba

River overflowed extensively by Porto Cercado.

There was as well a lag of several days between

stage changes in Barao de Melgaco and Porto

Cercado. The staff gage of the Ninhal site is located

on an arm of the Cuiaba, not on the main channel, and

at mid-distance between these two stations. In the

Ninhal, as in Porto Cercado, the river overflowed

during the flood period. Thus, the Porto Cercado

hydrogram is the best available estimator for the study

area taking into account a shorter lag with respect to

events recorded in Barao the Melgaco and, also, the

likely importance of backwater effects extending up to

the study area from the junction of the Cuiaba River

with one of its main affluent, the Sao Lourenco River.

4.2. Flood dynamics

During the flood, water invaded the floodplain on

the south side of the river (Fig. 1) and eventually

covered the whole Ninhal site with exception of the

Jacorutubinha levee emerging here and there. At the

forest site, the stream levels stayed lower than levee

elevation, but even then, instruments became flooded

from January to April 1999 when water levels on the

floodplain rose. However, a levee strip, reaching 70 m

wide, parallel to the river, remained dry during the

flood.

At the beginning of the flood, from December to

mid-January, the water level started to rise in the

floodplain. Water was contributed by direct rainfall

Fig. 2. Cuiaba River historical mean, maximum and minimum monthly stages. The mean monthly stages of 1998 are also shown for comparison.

These data are from the Cuiaba City staff gage and were obtained from ANEEL.

P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 61

and by the rising Cuiaba River. The Cuiaba Channel

cross-section is rectangular and since its levees are the

highest local elevations, they are the last lands to be

flooded and the first to emerge. Thus, water did not

directly overflow the riverbanks into the floodplain.

Rather, it flowed towards the floodplain through

various tie channels, some of which can be seen in

Fig. 1. As the floodplain filled up, the riparian forest

became inundated and the water eventually covered

much of the levees. During this period the water in the

channels usually ran from the river to the floodplain.

During the flood, the flow direction changed several

times in these channels, depending on the stage

differences between the floodplain and the river. In

consequence, during the flood, the floodplain and river

stages remained about equal. The floodplain acted as a

reservoir that stabilized the flood level. During the

receding water period, the river stage drops rapidly

causing the floodplain to empty and the flow in these

tie channels was in the direction of the river. During

the low-water phase these channels are dry or

the water they contain is stagnant. There were

numerous small channels flowing within the flood-

plain until the end of October 1999. When these dried,

the soil remained wet below the surface almost until

the end of November 1999, supporting abundant

terrestrial vegetation.

4.3. Hydrogeological framework

Groundwater was encountered from 2.0 to 5.6 m

depths in a sand bed below a sequence of alternating

clay and silt. When water was reached, it rose in the

bore-holes from about 0.4 to 3.4 m depending on the

location, indicating confined or semi-confined con-

ditions (Table 1). The piezometric levels stabilized

some 0.5–1.9 m lower than the Cuiaba River level (see

Fig. 4, November). It seems that there might be at least

two distinct groundwater layers and that will be discuss

later. This situation is different from the one that was

observed by Girard and Nunes da Cunha (1999) on

the right bank of the Cuiaba River, some 10 km

Fig. 3. Monthly stage readings of the Jacorutubinha channel at the study site. For comparison the daily stages (ANEEL records) for the Cuiaba

River at Barao de Melgaco (upstream) and Porto Cercado (downstream) are also shown.

P. Girard et al. / Journal of Hydrology 283 (2003) 57–6662

downstream, where a free water table was encoun-

tered. There, when the flood occurred, the groundwater

surface rose to ground or above-ground level. Here,

until the area became flooded, the presence of a clay–

silt cover over the sands impeded the groundwater

from physically rising and the observed piezometric

variations are then believed to be due to water pressure

increase within the confined groundwater layer.

In a general way, the variation of the Jacorutu-

binha’s surface water level appeared to control the

groundwater level fluctuations (Fig. 4), except in two

cases. When the instruments were flooded (Decem-

ber–April), there was very little difference between

the groundwater and river stages. After the flood, the

levels of P6, P7 and P8 as well as P12 and P16

continued to strictly follow the Jacorutubinha stage

until June 1999, when the groundwater, even though

closely following the river level remained lower than

the river. As for P4 and P5, even though their

piezometric head also decreased, the groundwater

level maintained itself well above the river level.

Furthermore, when the river started to rise towards the

end of October, their piezometric levels continued to

drop. One might speculate that they had continued to

do so until the area became flooded again, which had

not yet occurred by the end of December 1999.

4.3.1. Floodplain aquifer

Thus, in one case, illustrated by the synchronous

piezometric variations in P6, P7, P8, P12, and P16, it

seems that there was at least one continuous

groundwater-bearing sand layer at each site or even

one single groundwater body, namely the ‘floodplain

aquifer’. The piezometric levels in this aquifer were

always lower or equal to the river stage and recharge

could occur independently of surface flooding, as it

did when the river level rose in October–December

1999. However, flooding undoubtedly was the main

recharging event. In a previous study, Girard and

Pinto (2000) showed that, once the area became

flooded, the confining layer actually became saturated

and direct recharge of the confined water body by the

surface flood water could occur. At the beginning of

the dry period (June 1999), the aquifer levels became

de-coupled from the river one.

The local flow direction for the floodplain aquifer

was inferred from the piezometric measurements

(Fig. 1). At the forest site the exact groundwater

flow direction could not be deduced, as only two

piezometers were available. However, the apparent

flow direction from the piezometric record in P12 and

P16 during the dry period was in the SW quadrant. At

the Ninhal site, where the hydrographic network is

more complex, the mean true flow direction deduced

from P6, P7 and P8 (Fig. 1) was in the SE quadrant.

Along this direction the hydraulic gradient (in m/m)

varied from 1024 during the flood period to 1022

during low waters. In both cases local flow direction

was towards the southern floodplain away from the

Jacorutubinha channel. The groundwater may thus

have supported channel flow in the floodplain several

months after the flood was over and have contributed

in maintaining soil saturation almost 7 months after

the end of the previous flood.

This indicates that the river may interact with a

large volume of infill alluvium as Castro and

Hornberger (1991) described for mountain stream

channels. During the flood, a fraction of the surface

water could be temporarily stored within this alluvium

to be released during the dry period contributing to the

ecological stability of the river–floodplain system.

4.3.2. Bank aquifers

In the other case, the P4 and P5 groundwater

variations were not congruous with the fluctuations

recorded in the other piezometers and were not very

consistent between themselves indicating that the

aquifers in which they were implanted, namely ‘bank

aquifers’, were hydraulically disconnected from the

floodplain aquifer. Also, the flood appeared to be

the sole recharging event for the bank aquifers, as

Table 1

Piezometric rise in the well water levels upon installation in the

study area

Piezometer Water depth

upon boring (m)

Water rise (m) Water depth after

stabilization (m)

P4 3.75 2.61 1.14

P5 4.40 3.43 1.03

P6 2.65 1.32 1.33

P7 2.00 0.42 1.58

P8 2.50 1.04 1.46

P12 5.60 2.90 2.70

P16 5.25 2.72 2.53

See Fig. 1 for instrument locations. Depths are distances down

from the top of each well.

P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 63

the groundwater level did not rise along with the river

(October–December 1999) when the site was not

flooded. During the dry period, the piezometric levels

and river stage suggest that these aquifers drained into

the river at least until November 1999 (Fig. 4). From

this point onward, the river stage was higher than the

piezometric level in P5, suggesting a flow from

the river to the groundwater, which may explain why

the piezometric level in P5 ceased to decrease at that

point. The seepage outflow from the bank aquifers

appeared then to be significantly slower than that of

the floodplain aquifer and the flow direction still

remains unknown.

4.4. Dams, flood hydrology and groundwater

replenishment

Until recently, only small and mid-size dams

(#30 MW) were built on the Pantanal northern

tributaries. However at the end of 1999 the APM-

Manso dam (,200 MW), built on the Cuiaba River,

some 100 km upriver from the study site, started to

operate. At the end of 2002 an area of about 400 km2

had been flooded behind the dam constituting the dam

reservoir.

Dams have the capacity to alter the flow and even

flood regime depending on their size, type and also

numbers. The recent works of the World Commission

on Dams (WCD, 2000) leave little doubt about that.

The operation of large dams such as the APM-Manso

generally maintain the mean discharge of a river but

alter the minimum and maximum discharge to preset

values leading to a reduction of flood peaks. The main

expected impact of reduce flood peaks is a diminution

of the flooding in term of surface area and duration

(USGS, 1999; WCD, 2000).

The annual flood replenishes groundwater bodies

at the study area, sustaining the flow in floodplain

Fig. 4. The Jacorutubinha stage and piezometric variation during the study period. Instruments are localized in Fig. 1. All measurements are

referenced to an arbitrary datum. The insert in the figure shows the Jacorutubinha stage and piezometric variation at the site lacking a bird

nesting area.

P. Girard et al. / Journal of Hydrology 283 (2003) 57–6664

channels and also soils saturation, which in turn

contributes to the ecological stability in the flood-

plain. In the Ninhal site, recharge appears to occur

where the groundwater-bearing sand beds outcrop.

Thus, following Darcy’s law, annual recharge volume

will depend, at first approximation, on the area of

outcropping sand beds covered by flood waters, depth

of the flood waters, duration of inundation and the

hydraulic conductivity of the sand beds. Of these

variables, the first three depend on the flood peak, and

a reduction in flood peak will undoubtedly result in

less groundwater replenishment eventually compro-

mising ecological stability.

Furthermore, flooding occurs through tie channels

linking the floodplain and the main river channel.

During the low-waters phase, the bottoms of these

channels are generally exposed and well above the

main river surface. The altitude of the tie channels

bottom varies from one another. However, for the

main river water to enter the tie channels and flooding

of the flood plain to proceed, the main river stage has

to rise above a discrete level that varies from one tie

channel to another. Reduction of the flood peak and

duration may also reduce connectivity between the

floodplain and the main river channel.

5. Conclusion

The results of this study show that the flooding of

the Cuiaba floodplain in the Ninhal Corutuba area is

critically dependent on the river stage. As the river is

flanked by levees impeding direct access to the

floodplain, the water only invades it by flowing

through the tie channels. These only start to flow when

the river stage rises about a certain level.

As well, the flooding is also the main ground-

water replenishment mechanism. Groundwater

recharge is important to maintain floodplain channel

flow during the low waters when they do not

receive direct contribution of the Cuiaba waters

through the tie channels. Groundwater flow also

maintains soil saturation in the depressed parts of

the floodplain.

The recent construction of the Manso hydroelectric

facility, which will regulate the Cuiaba stages,

possibly reducing flood peaks may also alter the

floodplain hydrology. First, shallower floodwaters are

likely to reduce replenishment of the groundwater

bodies in the vicinity of the Cuiaba River. Second,

reduced flood peaks may result in a diminution of the

hydrological connectivity in the area. Both effects

would eventually lead to dryer conditions in the

floodplain during the low waters.

The construction of more reservoirs is planned in

other Pantanal tributaries. These works may also have

the capacity of reducing the flood peaks. It is

important to verify to what extent the observations

made in the Cuiaba floodplain apply to other Pantanal

sub-watersheds.

Acknowledgements

The authors wish to acknowledge the financial

support of the Pantanal Ecology Project (Max Planck

Institute for Limnology/Biosciences Institute—Fed-

eral University of Mato Grosso State) part of the

SHIFT (Studies of Human Impacts on Forests and

Floodplains in the Tropics) program a bilateral

technico-scientific Brazil – Germany cooperation

(CNPq-IBAMA-DLR). The authors are also grateful

to CAPES. Finally the authors thank the reviewers for

their constructive comments.

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