River–Groundwater Interactions in the Brazilian Pantanal. the Case of the Cuiaba´ River

Journal of Hydrology 283 (2003) 57–66 www.elsevier.com/locate/jhydrol

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 Biocieˆncias, 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 (Junk, 2000). This land is still in a rather pristine state and its ecological integrity is tightly linked to The Pantanal wetland is a vast evaporation plain its hydrology (Da Silva, 2000). Recently, however, and sediment accumulation surface (Junk et al., environmental disturbances such as deforestation due 2003) occupying an immense sedimentary to the expansion of the agro-industry and the depression. It is located south of the Amazon Basin consequent erosion and sedimentation have and east of the Andes, in the Upper Paraguay River increased. A new threat, the disruption of the Basin. Annually, the Upper Paraguay River and fundamental seasonal flood pulse by engineering its tributaries flood an area of about 140,000 km2 works such as waterways and large dams, is also emerging. One of these dams, which started operat- * Corresponding author. Fax: þ55-65-615-8264. ing at the end of 1999, is now regulating the flux of E-mail address: [email protected] (P. Girard). 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 58 P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 waters to the Pantanal—the largest river after the is now extensively used for agriculture and cattle Paraguay River itself. ranching. Next, there is the ‘Depression’ at altitudes In spite of the environmental impact study that was ranging from 180 to 250 m. This is a small region with performed before the dam construction, the body of generally steep slopes that is covered by a dense forest knowledge concerning the flood hydrology of the locally called ‘cerradaõ’. The last unit is the Pantanal, Cuiaba´ River in the Pantanal is still small. The most from less than 100 to 180 m altitude; it is about half important study was realized by the Conservation the size of the Plateau. It is a low relief plain with Plan for the Upper Paraguay Basin Project (PCBAP, hydraulic gradient not exceeding 15 cm/km. Many 1997), whereby the available hydrological data were large rivers, such as the Paraguay, the Cuiaba´, the Saõ inventoried and summarized. Hamilton et al. (1996) Lourenço, the Piquiri, the Taquari, and the Negro also studied the seasonality of Pantanal rivers in cross this vast plain. This complex hydrographic relation to flood extent. However there is still no network in conjunction with diverse soil types gives description of how the flooding proceeds and how the rise to a variety of landscapes within the Pantanal. river and groundwater interact. These large sub-units are subjected to different The aims of this paper are to contribute to the hydrological conditions, and different plant commu- hydrological knowledge of a reach of the Cuiaba´ nities characterize them. River located within the Pantanal, to describe water The climate of the Pantanal is marked by a flow between the river main channel and its adjacent pronounced dry season from May to September and flood plain, and relations between the surface water a rainy season from October to April. Mean monthly and the groundwater near the river. 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 2. Study area the temperature to drop as far as 0 8C. Annual rainfall decreases from 1250 mm in the northern Pantanal to The study area is located near a colonial bird 1089 mm in the south. Evapotranspiration, ranging nesting area, locally known as Ninhal Corutuba from 1100 to 1300 mm yearly (Hamilton et al., 1997; (Fig. 1). This Ninhal is on the margin of the Cuiaba´ Ponce, 1995), surpasses precipitation during at least 6 River in the northern portion of the Pantanal. In the months per year. Mean monthly air humidity varies in next paragraphs, the Pantanal, the Cuiaba´ River at the northern Pantanal from 84% during the rainy proximity of the study site, and the Ninhal are briefly season to below 60% at the end of the dry season, described. when the floodplain is dry (Tarifa, 1986). There is an emerging body of literature describing The annual flooding is caused by the sharp gradient the Pantanal (Ada´moli, 1981; Alvarenga et al., 1984; contrast between the Depression and the plain Da Silva, 2000; Da Silva et al., 2001; Hamilton et al., (Carvalho, 1986). Some geomorphological constric- 1996; Junk et al., 2003; PCBAP, 1997; RADAM- tions, namely rock outcrops, along the Paraguay River BRASIL, 1982) from which the following description reduce even more the ability of the river to drain away was assembled. The Pantanal is located along the floodwaters. In addition, local rainfall on the flood- course of the Upper Paraguay River, an important plains drains slowly, enhancing flooding. The Panta- tributary of the Parana´, one of earth’s largest rivers. nal is essentially a huge, gently sloped basin that The Upper Paraguay drains an area close to half a receives runoff from an upland watershed—the million square kilometers of which two thirds are in Plateau—twice its size and slowly releases the flood Brazil, in the states of Mato Grosso and Mato Grosso pulse of those waters through a single, downstream do Sul. According to topographical elevation this channel, the Paraguay River (Ponce, 1995). basin can be subdivided in three physiographic units. The annual flood pulse is mono-modal and presents First, the ‘Planalto’ or ‘Plateau’, 250–750 m a.s.l, is temporal and spatial variations. Along the main river the headwater region. A flat undulating plain con- channels the annual pulse is sharp and well defined, stitutes most of the Plateau and it is covered by a more driven mainly by river overflow. Farther from or less open savanna—locally called ‘cerrado’—that important channels, the flood pulse is more attenuated P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 59

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

(Penha et al., 1999). As the surface slope along the nesting area. At both sites staff gages were installed. Paraguay is about 3–5 cm/km, and because of the They consisted of four wood posts standing about 1 m rock outcrops mentioned earlier, the flood pulse above the ground level installed in a stairway fashion moves slowly southwards and there is a lag of 4–6 from the top of the left bank levee. The water levels months between the flood peaks in the north and in the were measured from the top of the post with a south. Most of the water enters the northern Pantanal, conventional meter. The accuracy is ^0.001 m. as the three major contributors are the Paraguay, the Standpipe piezometers were installed in both sites. Saõ Lourenço and the Cuiaba´ Rivers. The flood pulse They consisted of PVC tubes ðf ¼ 32 mmÞ that were maintains the biodiversity and health of the Pantanal manually slotted in the lower 0.5 m. The slotted ecosystem (Da Silva and Esteves, 1993; Espindola section was covered with a fine mosquito screen. et al., 1996; Penha et al., 1998, 1999; Resende et al., These instruments were inserted in manually bored 1996; Strussman, 1991). holes ðf ¼ 60 mmÞ: The slotted section was installed The vast fluviolacustrine plain of the Cuiaba´ River within the aquifer sand formation. The slotted section consists of actual alluvium that forms fluvial islands, was coated with local clean quartz sand and covered marginal levees, and bars. Grain size varies from with a concrete plug to prevent floodwater infiltration. sands to clays and most abundant are sandy silt and Static level within the piezometer was measured from clay–silt beds that are often intercalated. the instrument top with a sound indicator. The The Ninhal Corutuba is located on the left margin accuracy is ^0.005 m. Five piezometers were of the Cuiaba´ River (Fig. 1). There, the Cuiaba´ installed at the Ninhal (P4–P8) and two at the forest Channel splits in two and its smaller arm is called (P12 and P16) site. Approximate locations of all ‘Jacorutubinha’. The Ninhal is located where the instruments are given in Fig. 1. Jacorutubinha meets the Cuiaba´ main Channel At each site, the relative elevations of the (16828018700S and 56807053600W). In this region the instrument tops were determined. The highest instru- Cuiaba´ River flows over its own fluviolacustrine ment at each site (the staff gage on the top of the sediments. Along the river course there is a series of Jacorutubinha levee) was arbitrarily assigned an small lakes seasonally linked to the Cuiaba´ and elevation of 100 m. A transparent tube full of water abandoned meanders. There is also a high density of was then used to find the elevation of the other tie channels in which the water may flow both ways instruments. and other channel types that present a parallel Readings from these instruments were obtained at drainage pattern. least once a month from November 1998 to December 1999.

3. Material and methods 4. Results and discussion 3.1. Institutional data acquisition 4.1. Hydrological framework The Cuiaba´ River stages were acquired from the ANEEL (Brazil’s National Agency for Electrical There are no historical fluviometric records in the Energy—www.aneel.gov.br). The complete daily Jacorutubinha channel. However, the Cuiaba´ River readings from the Cuiaba´, Baraõ de Melgaço and has several fluviometric stations. The longest record, Porto Cercado region, from the beginning of 1998 starting in 1933, is from the Cuiaba´ City station some until end of March 1999, were obtained. 75-km upstream of the study site. The Cuiaba´ is a strongly seasonal river and Fig. 2 shows the hydro- 3.2. Fluviometric and piezometric data graph of the historical mean monthly stages as well as at the study site the variation of the historical mean monthly minimum and maximum. The mean monthly stages for the year There were two study locations, one in the Ninhal 1998 are also displayed showing that this was an and one in a nearby riparian forest lacking a bird atypical year, as the river stages were unusually low. P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 61

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.

This was also the case in the Pantanal at the Baraõde events recorded in Baraõ the Melgaço and, also, the Melgaço and Porto Cercado stations. likely importance of backwater effects extending up to In the Ninhal, the river stages were recorded only the study area from the junction of the Cuiaba´ River once a month, but daily stages available upstream with one of its main affluent, the Saõ Lourenço River. (Baraõ de Melgaço) and downstream (Porto Cercado) on the Cuiaba´ River, provide a good indicator of the 4.2. Flood dynamics hydrograph shape in the Ninhal. Compared to the Baraõ de Melgaço record, the stage at Porto Cercado During the flood, water invaded the floodplain on was smoother during flood time (Fig. 3), due to the the south side of the river (Fig. 1) and eventually fact that, unlike in Baraõ de Melgaço, the Cuiaba´ covered the whole Ninhal site with exception of the River overflowed extensively by Porto Cercado. Jacorutubinha levee emerging here and there. At the There was as well a lag of several days between forest site, the stream levels stayed lower than levee stage changes in BaraõdeMelgaço and Porto elevation, but even then, instruments became flooded Cercado. The staff gage of the Ninhal site is located from January to April 1999 when water levels on the on an arm of the Cuiaba´, not on the main channel, and floodplain rose. However, a levee strip, reaching 70 m at mid-distance between these two stations. In the wide, parallel to the river, remained dry during the Ninhal, as in Porto Cercado, the river overflowed flood. during the flood period. Thus, the Porto Cercado At the beginning of the flood, from December to hydrogram is the best available estimator for the study mid-January, the water level started to rise in the area taking into account a shorter lag with respect to floodplain. Water was contributed by direct rainfall 62 P. Girard et al. / Journal of Hydrology 283 (2003) 57–66

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 Baraõ de Melgaço (upstream) and Porto Cercado (downstream) are also shown. and by the rising Cuiaba´ River. The Cuiaba´ Channel the water they contain is stagnant. There were cross-section is rectangular and since its levees are the numerous small channels flowing within the flood- highest local elevations, they are the last lands to be plain until the end of October 1999. When these dried, flooded and the first to emerge. Thus, water did not the soil remained wet below the surface almost until directly overflow the riverbanks into the floodplain. the end of November 1999, supporting abundant Rather, it flowed towards the floodplain through terrestrial vegetation. various tie channels, some of which can be seen in Fig. 1. As the floodplain filled up, the riparian forest 4.3. Hydrogeological framework became inundated and the water eventually covered much of the levees. During this period the water in the Groundwater was encountered from 2.0 to 5.6 m channels usually ran from the river to the floodplain. depths in a sand bed below a sequence of alternating During the flood, the flow direction changed several clay and silt. When water was reached, it rose in the times in these channels, depending on the stage bore-holes from about 0.4 to 3.4 m depending on the differences between the floodplain and the river. In location, indicating confined or semi-confined con- consequence, during the flood, the floodplain and river ditions (Table 1). The piezometric levels stabilized stages remained about equal. The floodplain acted as a some 0.5–1.9 m lower than the Cuiaba´ River level (see reservoir that stabilized the flood level. During the Fig. 4, November). It seems that there might be at least receding water period, the river stage drops rapidly two distinct groundwater layers and that will be discuss causing the floodplain to empty and the flow in these later. This situation is different from the one that was tie channels was in the direction of the river. During observed by Girard and Nunes da Cunha (1999) on the low-water phase these channels are dry or the right bank of the Cuiaba´ River, some 10 km P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 63

Table 1 aquifer’. The piezometric levels in this aquifer were Piezometric rise in the well water levels upon installation in the always lower or equal to the river stage and recharge study area could occur independently of surface flooding, as it Piezometer Water depth Water rise (m) Water depth after did when the river level rose in October–December upon boring (m) stabilization (m) 1999. However, flooding undoubtedly was the main recharging event. In a previous study, Girard and P4 3.75 2.61 1.14 Pinto (2000) showed that, once the area became P5 4.40 3.43 1.03 flooded, the confining layer actually became saturated P6 2.65 1.32 1.33 P7 2.00 0.42 1.58 and direct recharge of the confined water body by the P8 2.50 1.04 1.46 surface flood water could occur. At the beginning of P12 5.60 2.90 2.70 the dry period (June 1999), the aquifer levels became P16 5.25 2.72 2.53 de-coupled from the river one. See Fig. 1 for instrument locations. Depths are distances down The local flow direction for the floodplain aquifer from the top of each well. was inferred from the piezometric measurements (Fig. 1). At the forest site the exact groundwater downstream, where a free water table was encoun- flow direction could not be deduced, as only two tered. There, when the flood occurred, the groundwater piezometers were available. However, the apparent surface rose to ground or above-ground level. Here, flow direction from the piezometric record in P12 and until the area became flooded, the presence of a clay– P16 during the dry period was in the SW quadrant. At silt cover over the sands impeded the groundwater the Ninhal site, where the hydrographic network is from physically rising and the observed piezometric more complex, the mean true flow direction deduced variations are then believed to be due to water pressure from P6, P7 and P8 (Fig. 1) was in the SE quadrant. increase within the confined groundwater layer. Along this direction the hydraulic gradient (in m/m) In a general way, the variation of the Jacorutu- varied from 1024 during the flood period to 1022 binha’s surface water level appeared to control the during low waters. In both cases local flow direction groundwater level fluctuations (Fig. 4), except in two was towards the southern floodplain away from the cases. When the instruments were flooded (Decem- Jacorutubinha channel. The groundwater may thus ber–April), there was very little difference between have supported channel flow in the floodplain several the groundwater and river stages. After the flood, the months after the flood was over and have contributed levels of P6, P7 and P8 as well as P12 and P16 in maintaining soil saturation almost 7 months after continued to strictly follow the Jacorutubinha stage the end of the previous flood. until June 1999, when the groundwater, even though This indicates that the river may interact with a closely following the river level remained lower than large volume of infill alluvium as Castro and the river. As for P4 and P5, even though their Hornberger (1991) described for mountain stream piezometric head also decreased, the groundwater channels. During the flood, a fraction of the surface level maintained itself well above the river level. water could be temporarily stored within this alluvium Furthermore, when the river started to rise towards the to be released during the dry period contributing to the end of October, their piezometric levels continued to ecological stability of the river–floodplain system. drop. One might speculate that they had continued to do so until the area became flooded again, which had 4.3.2. Bank aquifers not yet occurred by the end of December 1999. In the other case, the P4 and P5 groundwater variations were not congruous with the fluctuations 4.3.1. Floodplain aquifer recorded in the other piezometers and were not very Thus, in one case, illustrated by the synchronous consistent between themselves indicating that the piezometric variations in P6, P7, P8, P12, and P16, it aquifers in which they were implanted, namely ‘bank seems that there was at least one continuous aquifers’, were hydraulically disconnected from the groundwater-bearing sand layer at each site or even floodplain aquifer. Also, the flood appeared to be one single groundwater body, namely the ‘floodplain the sole recharging event for the bank aquifers, as 64 P. Girard et al. / Journal of Hydrology 283 (2003) 57–66

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. the groundwater level did not rise along with the river tributaries. However at the end of 1999 the APM- (October–December 1999) when the site was not Manso dam (,200 MW), built on the Cuiaba´ River, flooded. During the dry period, the piezometric levels some 100 km upriver from the study site, started to and river stage suggest that these aquifers drained into operate. At the end of 2002 an area of about 400 km2 the river at least until November 1999 (Fig. 4). From had been flooded behind the dam constituting the dam this point onward, the river stage was higher than the reservoir. piezometric level in P5, suggesting a flow from Dams have the capacity to alter the flow and even the river to the groundwater, which may explain why flood regime depending on their size, type and also the piezometric level in P5 ceased to decrease at that numbers. The recent works of the World Commission point. The seepage outflow from the bank aquifers on Dams (WCD, 2000) leave little doubt about that. appeared then to be significantly slower than that of The operation of large dams such as the APM-Manso the floodplain aquifer and the flow direction still generally maintain the mean discharge of a river but remains unknown. alter the minimum and maximum discharge to preset values leading to a reduction of flood peaks. The main 4.4. Dams, flood hydrology and groundwater expected impact of reduce flood peaks is a diminution replenishment of the flooding in term of surface area and duration (USGS, 1999; WCD, 2000). Until recently, only small and mid-size dams The annual flood replenishes groundwater bodies (#30 MW) were built on the Pantanal northern at the study area, sustaining the flow in floodplain P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 65 channels and also soils saturation, which in turn likely to reduce replenishment of the groundwater contributes to the ecological stability in the flood- bodies in the vicinity of the Cuiaba´ River. Second, plain. In the Ninhal site, recharge appears to occur reduced flood peaks may result in a diminution of the where the groundwater-bearing sand beds outcrop. hydrological connectivity in the area. Both effects Thus, following Darcy’s law, annual recharge volume would eventually lead to dryer conditions in the will depend, at first approximation, on the area of floodplain during the low waters. outcropping sand beds covered by flood waters, depth The construction of more reservoirs is planned in of the flood waters, duration of inundation and the other Pantanal tributaries. These works may also have hydraulic conductivity of the sand beds. Of these the capacity of reducing the flood peaks. It is variables, the first three depend on the flood peak, and important to verify to what extent the observations a reduction in flood peak will undoubtedly result in made in the Cuiaba´ floodplain apply to other Pantanal less groundwater replenishment eventually compro- sub-watersheds. mising ecological stability. Furthermore, flooding occurs through tie channels linking the floodplain and the main river channel. Acknowledgements During the low-waters phase, the bottoms of these channels are generally exposed and well above the The authors wish to acknowledge the financial main river surface. The altitude of the tie channels support of the Pantanal Ecology Project (Max Planck bottom varies from one another. However, for the Institute for Limnology/Biosciences Institute—Fed- main river water to enter the tie channels and flooding eral University of Mato Grosso State) part of the of the flood plain to proceed, the main river stage has SHIFT (Studies of Human Impacts on Forests and to rise above a discrete level that varies from one tie Floodplains in the Tropics) program a bilateral channel to another. Reduction of the flood peak and technico-scientific Brazil–Germany cooperation duration may also reduce connectivity between the (CNPq-IBAMA-DLR). The authors are also grateful floodplain and the main river channel. to CAPES. Finally the authors thank the reviewers for their constructive comments.

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