HYDROLOGICAL PROCESSESHydrol. Process. 23, 3127–3130 (2009)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7408

Preface

Flow and sediment dynamics of large rivers

J. L. Guyot1* and D. E. Walling2*1 Institut de Recherche pour le Developpement—IRD, UMR 154 LMTG, CP 7091 Lago Sul, 71619-970 Brasilia, DF, Brazil

2 Department of Geography, University of Exeter, Exeter, EX4 4RJ, UK

Much of the recent hydrological research has focussedon small catchments or drainage basins, and such researchhas made a major contribution to the current understand-ing of hydrological processes and their key controls andthe sensitivity of these processes to external forcing asso-ciated with the many facets of global change. However,there is a need to study hydrological processes at a rangeof spatial scales, and the behaviour of large rivers isattracting increasing attention, as they represent an impor-tant component of the earth system (e.g. Gupta, 2008).They provide important routes for the transfer of water,sediment and solutes and associated pollutants from theland to the oceans. Furthermore, they integrate the hydro-logical response of very large areas, and changes in theirbehaviour in response to global change are an importantindicator of changes in the hydrological response of theearth system. Understanding the behaviour of large riversalso brings the need for different approaches, both to datacollection and to the spatial integration of the hetero-geneity frequently contained within their large drainagebasins. When modelling large rivers and their drainagebasins, there is a need to consider carefully the mostappropriate temporal and spatial scale for model develop-ment, as the response of such rivers is commonly highlydamped by the extended travel times, and downstreamchannel storage and routing exert a key influence on theirflow regimes. Many of the large rivers of the world arefound in developing countries and lack well-developedmonitoring programmes. Work on such rivers thereforefrequently involves collaboration between national teamsand overseas researchers, as, for example, demonstratedby the work of French researchers from the Institut deRecherche pour le Developpement (IRD, ex ORSTOM),in South America and tropical Africa.

The need for a distinctive approach to the study oflarge rivers and their drainage basins was usefully demon-strated by the staging of the International Symposiumon Hydrological and Geochemical Processes in LargeScale River Basins held in Manaus, Brazil in 1999. The

* Correspondence to: J. L. Guyot, Institut de Recherche pour leDeveloppement—IRD, UMR 154 LMTG, CP 7091 Lago Sul, 71619-970Brasilia, DF, Brazil. E-mail: jean-loup.guyot@ird.fr* Correspondence to: D. E. Walling, Department of Geography, Univer-sity of Exeter, Exeter, EX4 4RJ, UK. E-mail: d.e.walling@exeter.ac.uk

venue provided an opportunity to highlight recent workon the Amazon, but its coverage was much wider andincluded reports of work on many of the world’s largerivers. Although there has been no direct successor tothis symposium, large rivers and their drainage basinsstill represent an important focus of international inter-est. This collection of papers, many of which are authoredby participants in the Manaus symposium, aims to pro-vide information on current issues and approaches in theongoing research on large rivers. The emphasis is on largerivers in developing countries and a significant proportionof the papers deals with the Amazon and other large riversin South America, although work on several large Africanrivers is also included. By virtue of its important collab-orative work on large rivers in developing countries, thework of the IRD also receives particular attention.

The 18 papers included in this collection highlightseveral important aspects relating to the study of theflow and sediment dynamics of large rivers and theirdrainage basins and their sensitivity to global change.The first three papers emphasise the distinctive natureof the flow of large rivers and the problems associatedwith measuring the discharge of such rivers. The paperby Laraque et al. (2009b) reports the results of detailedobservations of the mixing of the waters of the Negroand Solimoes Rivers in the Amazon basin, which oftenfeatures in tourist excursions from Manaus. At thetime of the measuring campaign, these two rivers werecharacterized by different flow velocities (0Ð3 vs 1 ms�1), different conductivities (8 vs 80 µs cm�1 at 25 °C),different pH (5Ð5 vs 7Ð0) and different turbidities (5 vs80 NTU) as well as different temperatures. Becauseof their higher density, the waters of the SolimoesRiver slid under those of the Negro River and completemixing was only achieved 100 km downstream andtook 30 h. Again, dealing with the Amazon, Kosuthet al. (2009) report an investigation of the physicalinfluence of sea tides on the flow in the lower reachesof the Amazon River. The tidal influence is currentlyfelt more than 1000 km upstream from the sea andthe physical processes of wave propagation result in acyclic pattern of water storage and release, which in turnaffects downstream sediment transport and associateddeposition. The problems of measuring the discharge oflarge rivers such as the Amazon, where river widths

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3128 J. L. GUYOT AND D. E. WALLING

can exceed several kilometres and water depths can bein excess of 50 m, and the need for new approachesare reviewed by Filizola et al. (2009) with particularreference to measurements undertaken at the Manucapurasection on the Solimoes River, a tributary of the AmazonRiver. ADCP measurements were successfully used atthis section to measure discharges of the order of100 000 m3 s�1, and their potential advantages in termsof reproducibility, speed of measurement and reductionin the rating curve scatter are clearly demonstrated.

Three further papers focus on specific aspects ofthe morphological features of large river basins andtheir hydrological functioning. Mahe et al. (2009) reportresults from recent investigations of the inner delta ofthe River Niger, which covers more than 70 000 km2

and intercepts flow from an upstream catchment of249 000 km2. The average flooded area is calculated tobe ca. 24 000 km2 and water losses from this area resultin a reduction of the average discharge at the inflowto the inner delta of 1490 m3 s�1 to 900 m3 s�1 atthe outflow. This represents an overall loss of ca. 40%,which is equivalent to ca. 18Ð7 km3. Inundation of vastareas during the annual flood is also a key feature of theAmazon basin in Bolivia. Here, the upper reaches of theMadeira River drain a basin of ca. 900 000 km2, and asmuch as 150 000 km2 may be inundated during the wetseason. The need to study the spatio-temporal dynamicsof these inundated areas and the inter-annual variability oftheir extent introduces particular problems in this regionas they are difficult to access, and a study reported byBourrel et al. (2009) that focuses on the Mamore Riverprovides a valuable demonstration of the potential forusing a combination of classic and radar satellite imageryto document the evolution of the inundated area duringthe flood season. In this region, inundation was shownto be associated with two main mechanisms, the relativeimportance of which varied from year to year. The firstmechanism was exogenous and involved flow of wateracross the floodplains from the flood in the main river,whilst the second was endogenous and reflected risinggroundwater levels fed by local precipitation. In anothercontribution relating to the Amazon River, Irion et al.(2009) report an investigation of the impact of Quaternarysea level changes on the evolution of the contemporarychannel and floodplain system in the lower reaches ofthis river system. A programme of coring and acousticsurveys was employed to reveal how the influence of lowsea levels associated with the last glacial maximum werefelt more than 1500 km upstream of the river mouth.River levels fell by at least 30 m at Manaus, at this time,causing incision, increased channel slope and increasedbedload transport capacity. Subsequent sea level risecaused a backwater effect extending far upstream andresulting in aggradation, the creation of the Ria lakesand the eventual formation of the floodplains or varzeaabout 5000 years ago, when the sea level approached itscurrent level.

The various scale-related issues associated with char-acterizing and modelling the flow and sediment response

of very large river basins are addressed by the nextfour papers. The scale problems related to assemblingrepresentative information on the river network and sub-catchment boundaries of a large river basin in a develop-ing country, as an essential prelude to the applicationof a distributed physically based model, are usefullyhighlighted by Seyler et al. (2009). These authors reportwork on integrating available digital elevation models(DEMs) with information from topographic maps andremote sensing imagery within a geographical informa-tion system (GIS) for the Amazon basin, which has ca.250 sub-watersheds linked to flow measuring stations.The 700 000 km2 River Negro basin is used as a casestudy to demonstrate the problems encountered and thepotential of the various data sources. Conway and Mahe(2009) focus on the application of a conceptual waterbalance model in modelling monthly river flows in thetributaries of the Parana River in South America and theRiver Niger in Africa, using global datasets of rainfall,potential evapotranspiration (PE) and soil available watercapacity with a 0Ð5°latitude and longitude resolution. Thisstudy emphasises the sensitivity of parameter values andmodel performance to the input data sets and particu-larly the procedure used to estimate PE. The two riversdemonstrated contrasting responses to recent changes inrainfall, with the flows of the Parana River increasingby ca. 28% and the flows of the Niger River decreasingby ca. 34%. The problem of modelling the relationshipbetween suspended sediment concentration and water dis-charge and the associated hysteresis for a large riversystem is addressed by Picouet et al. (2009), using datafrom the 71 800-km2 basin of the Upper Niger River inWest Africa. Two models were developed. The first wasbased on simple empirical relationships between sedimentconcentration and flow and provided separate relation-ships for the rising stage and recession periods of theannual flood. The second model, a lumped conceptualmodel, incorporated contributions from two distinct sed-iment reservoirs or stores. The first represents hillslopeerosion during the wet season and the second representssediment contributed by bank erosion and the remobi-lization of sediment deposits from within the channelnetwork. Both models were successfully used to describethe time evolution of suspended sediment concentrationsduring the annual flood at the Banankoro gauging stationon the Upper Niger. Finally, flood regionalization pro-cedures for large river basins are considered by Versianiet al. (2009). In this case, the 50 870-km2 basin of theSao Francisco River in Brazil provides a case study. Atwo-component extreme value (TCEV) distribution func-tion was applied to the annual maximum flood series andthis was successfully used to define two homogeneousregions for flood prediction purposes.

Important information on the sediment loads of severalof the world’s large rivers, including the Amazon, theCongo and the Parana-Paraguay Rivers are providedby the next four papers. Filizola (2009) present theresults of an analysis of the Brazilian national data setfor the Amazon Region, which comprises more than

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 3127–3130 (2009)DOI: 10.1002/hyp

FLOW AND SEDIMENT DYNAMICS OF LARGE RIVERS 3129

2500 samples collected from 60 measuring stations.Sampling procedures and methods of flux calculationare discussed and estimates of mean annual suspendedsediment load are presented for 49 stations within theAmazon basin to provide valuable information on thespatial variability of sediment yield across the basin.In addition, information on intra-annual variability ofsediment transport is presented for key sites. On thebasis of these data, the mean annual suspended sedimentload of the River Amazon at Obidos was estimatedto be 556 million t year�1. Estimates of the annualmaterial fluxes transported by the main tributaries ofthe Congo River, the largest river basin on the Africancontinent, are presented by Laraque et al. (2009a). Bycoupling available measurements with a physiographiczonation of the basin, they have produced estimates of thesuspended sediment (SS) and total dissolved solids (TDS)loads for individual tributaries, and this information hasbeen used to produce maps of the specific sedimentand dissolved solids yields for the main sub-catchmentsof the Congo Basin. Throughout the Congo basin, theTDS yields commonly exceed the SS yields and averageyields at the basin outlet are estimated to be 12Ð1 tkm�2 year�1 and 8Ð3 t km�2 year�1 respectively. Thevast forested depression of the ‘Cuvette Centrale’, whichoccupies almost half the Congo basin exerts an importantinfluence on material transport, as sediment productionis very low, but high concentrations of dissolved organicmaterial (DOM) are found. In some flooded zones, DOMconcentrations in the rivers can reach 80 mg l�1 andDOM can account for >50% of the TDS. The potentialimportance of tectonic activity in controlling sedimentfluxes in large river basins is emphasized by Baby et al.(2009) in a study of the Madeira River in Bolivia, amajor tributary of the Amazon. The Madeira River drainspart of the orogenic wedge of the eastern Andes andsubsequently flows across a subsiding foreland basinrepresented by the Beni plain. The sediment flux fromthe upper part of the basin (173 000 km2), which drainsthe Cordillera Oriental and the Subandean foothills isestimated to be in the range of 500–600 million t year�1,representing a specific sediment yield of ca. 3200 t km�2

year�1. However, it is estimated that 270 million t year�1

are deposited in the subsiding basin, so that only ca.46% of the sediment load mobilized within the upperbasin is transported downstream into the Amazon system.The final paper of this group by Amsler and Drago(2009) directs attention to recent changes in the sedimentloads of the Parana-Paraguay Rivers. A comparison ofdata collected in the 1970s with those collected inthe 1990s provides clear evidence of changes in thesediment loads transported by this large river system.Increased precipitation and runoff across parts of theParana-Paraguay system have caused increased erosionand sediment mobilization, but in some rivers this hasbeen offset by sediment trapping by dams, resulting inreduced sediment loads. In the case of the Upper Parana,the sediment load was found to have decreased by 60%due to sediment trapping by dams, including the Itaipu

and Yacireta dams. However, the sediment load of theBermejo River, a major tributary of the Paraguay Riverwas estimated to have increased by 85%, resulting inan overall increase in the sediment load of the MiddleParana River, below the confluence of the Upper Paranaand Paraguay Rivers by 35%. In the 1970s, the BermejoRiver contributed 60% of the sediment load of the MiddleParana, but in the 1990s this increased to 90%.

The final four papers are concerned with the influ-ence of ENSO and related phenomena on the longer-termvariability of rainfall and the hydrological response ofrivers in the Amazon basin and in Argentina, Ecuadorand tropical Africa. The contribution by Marengo (2009)focuses on the Amazon basin and uses both rainfall anddischarge records to explore longer-term climatic vari-ability across the region since the 1920s. No systematiclong-term trend was detected, but clear evidence of cyclicvariability linked to ENSO events with a periodicity of20–30 years was detected. These cycles result in decadaland multi-decadal variations in the hydrology of thebasin. Links between long-term rainfall variability andchanges in regional circulation reflected by sea level pres-sure (SLP) gradients and sea surface temperature (SST)gradients within the Atlantic sector were explored. Abroader assessment of hydrological variability involvingboth the Amazon basin and several large tropical riverbasins in Africa is reported by Molinier et al. (2009).Analysis of long-term monthly mean rainfall and dis-charge records for about 50 rainfall and hydrometricstations in the Amazon basin provided clear evidenceof the influence of ENSO phenomena on the longer-term variability of Amazonian hydrological regimes. Thisinfluence may be strengthened by SST gradients in thetropical Atlantic. A similar analysis of several large riverbasins in tropical Africa, including the Senegal, Niger andCongo, demonstrated similar variability, although a com-parison of the Amazon and Congo, the two largest riversin the world, showed an inverse relationship. Overall, theinfluence of ENSO events was weaker in tropical Africa.In that region of the world, the hydrological variabilityappeared to be better explained by SST anomalies in theSouth Atlantic. A detailed investigation of the influenceof the ENSO phenomenon on the magnitude of monthlyrainfall over the coastal region of Ecuador is describedby Rossel and Cadier (2009). Multiple regression mod-els involving a 1-month lead time were successfully usedto predict monthly rainfall in the Guayaquil region, as afunction of precipitation, SSTs and meridional and zonalwind in the eastern equatorial Pacific during the previousmonth. The best predictor for February and March rain-fall was found to be the rainfall for the previous monthin the northern area of the coast. This might be combinedwith the meridian ITCZ migration. The best predictors forApril and May rainfall proved to be SST indices from thetropical Pacific close to the Ecuador coast, reflecting theimportant influence of ENSO and SST anomalies at theend of the rainy season. The contribution from Chavasseand Seoane (2009) looks in more detail at the increas-ing flow variability and the problems of flooding within

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 3127–3130 (2009)DOI: 10.1002/hyp

3130 J. L. GUYOT AND D. E. WALLING

the Upper Parana basin associated with ENSO events.Attention focuses on the 7500 km2 basin of the ChopimRiver, a tributary of the Iguazu River, which is one of themain sub-catchments of the Upper Parana River in Brazil.In order to explore possible flood problems associatedwith El Nino years, prediction techniques for forecast-ing flood discharges were developed. A rainfall–runoffmodel based on the Sacramento soil moisture accountingprocedure and employing daily rainfall data was success-fully validated for the Chopim basin, and this model wasused with past rainfall data series for El Nino and non-Nino years to forecast possible future runoff.

The collection of papers included in this special issuenecessarily provides only partial coverage of the manyinteresting problems and issues associated with the flowand sediment dynamics of large rivers and indeed ofthe world’s large rivers. However, it is hoped that itwill help promote further work in this general fieldand encourage further integration of knowledge andunderstanding obtained from large rivers in differentareas of the globe.

REFERENCES

Amsler ML, Drago EC. 2009. A review of the suspended sedimentbudget at the confluence of the Parana and Paraguay Rivers.Hydrological Processes 23: 3230–3235.

Baby P, Guyot JL, Herail G. 2009. Tectonic control of erosion andsedimentation in the Amazon Basin of Bolivia. Hydrological Processes23: 3225–3229.

Bourrel L, Phillips L, Moreau S. 2009. The dynamics of floods in theBolivian Amazon basin. Hydrological Processes 23: 3161–3167.

Chavasse DI, Seoane RS. 2009. Assessing and predicting the impact of ElNino Southern Oscillation (ENSO) events on runoff from the ChopimRiver basin, Brazil. Hydrologoical Processes 23: 3261–3266.

Conway D, Mahe G. 2009. River flow modelling in two large river basinswith non-stationary behaviour: the Parana and the Niger. HydrologicalProcesses 23: 3186–3192.

Filizola N, Guyot JL, Guimaraes V. 2009. Measuring the discharge ofthe Amazon River using Doppler technology (Manacapuru, Amazonas,Brazil). Hydrological Processes 23: 3151–3156.

Filizola N, Guyot JL. 2009. Suspended sediment yields in the Amazonbasin: an assessment using the Brazilian national data set. HydrologicalProcesses 23: 3207–3215.

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Kosuth P, Callede J, Laraque A, Filizola N, Guyot JL, Seyler P,Fritsch JM, Guimaraes V. 2009. Sea-tide effects on flows in the lowerreaches of the Amazon River. Hydrological Processes 23: 3141–3150.

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Marengo JA. 2009. Long-term trends and cycles in the hydrometeorologyof the Amazon basin since the late 1920s. Hydrological Processes 23:3236–3244.

Molinier M, Ronchail J, Guyot JL, Cochonneau G, Guimaraes V, deOliveira E. 2009. Hydrological variability in the Amazon drainagebasin and African tropical basins. Hydrological Processes 23:3245–3252.

Picouet C, Hingray B, Olivry JC. 2009. Modelling the suspendedsediment dynamics of a large tropical river: the Upper Niger Riverbasin at Banankoro. Hydrological Processes 23: 3193–3200.

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Seyler F, Muller F, Cochonneau G, Guimaraes L, Guyot JL. 2009.Watershed delineation for the Amazon sub-basin system usingGTOPO30 DEM and a drainage network extracted from JERS SARimages. Hydrological Processes 23: 3173–3185.

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Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 3127–3130 (2009)DOI: 10.1002/hyp