Temporal relations between meander deformation, water discharge and sediment ﬂuxes in the...

Meander deformation, water discharge and sediment flux 1

Copyright © 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms (in press)DOI: 10.1002/esp

Earth Surface Processes and LandformsEarth Surf. Process. Landforms (in press)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1394

Temporal relations between meander deformation,water discharge and sediment fluxes in thefloodplain of the Rio Beni (Bolivian Amazonia)E. Gautier,1* D. Brunstein,2 P. Vauchel,3 M. Roulet,3 O. Fuertes,4 J. L. Guyot,5 J. Darozzes6 andL. Bourrel71 University Paris 8, Dept. of Geography and CNRS Laboratoire de Géographie Physique, Meudon Cedex, France2 CNRS Laboratoire de Géographie Physique, Meudon Cedex, France3 Institut de Recherche pour le Développement, La Paz, Bolivia4 SENAMHI, La Paz, Bolivia5 Institut de Recherche pour le Développement, Lima, Peru6 Université Paul Sabatier, LMTG, Toulouse, France7 Institut de Recherche pour le Développement, Quito, Ecuador

AbstractThe Andean Cordillera and piedmont significantly influence river system and dynamics,being the source of many of the important rivers of the Amazon basin. The Beni River,whose upper sub-catchments drain the Andean and sub-Andean ranges, is a major tributaryof the Madeira River. This study examines the river in the south-western Amazonianlowlands of Bolivia, where it develops mobile meanders. Channel migration, meander-bendmorphology and ox-bow lakes are analysed at different temporal and spatial scales. The firstpart of this study was undertaken with the aim to link the erosion–deposition processes inthe active channel with hydrological events. The quantification of annual erosion and deposi-tion areas shows high inter-annual and spatial variability. In this study, we investigate theconditions of sediment exportation in the river in relation to three hydrological parameters(flood intensity, date of discharge peak and duration of the bank-full stage level). The secondpart of this study, focusing on the abandoned meanders, analyses the cutoff processes andthe post-abandonment evolution during 1967–2001. This approach shows the influence of theactive channel behaviour on the sediment diffusion and sequestration of the abandonedmeanders and allows us to build a first model of the contemporary floodplain evolution.Copyright © 2006 John Wiley & Sons, Ltd.

Keywords: meandering pattern; ox-bow lakes; Beni River; Bolivian Amazonia

Received 27 June 2005;Revised 23 March 2006;Accepted 10 April 2006

*Correspondence to: E. Gautier,University Paris 8, Dept. ofGeography and CNRSLaboratoire de GéographiePhysique, 1 place A. Briand,92195 Meudon Cedex, France.E-mail: [email protected]

Introduction

Meandering rivers have been the subject of abundant literature aiming to establish a classification of meander andevolutionary models (Blum et al., 2005; Brice, 1974; Bridge, 2003; Daniel, 1971; Dury, 1976; Hickin, 1974; Hooke,1984, 1995a; Hooke and Harvey, 1983; Schumm, 1963). Recently, models based on numerical simulations haveproposed a predictive approach of meander migration (Bridge, 1992; Darby and Delbono, 2002; Howard, 1996;Howard and Hemberger, 1991; Lancaster and Bras, 2002; Mosselman, 1998; Tucker et al., 2001, among others).

Among these numerous studies, several main themes are apparent, providing possible explanations for the variabil-ity of meander mobility. Some work has focused on the geometry of the channel: interrelations between the channelcurvature/width ratio and the migration rate (Hickin, 1974; Hickin and Nanson, 1975; Richards, 1982). Both labora-tory flumes and field observations highlight the effectiveness of secondary flows, making the shear stress higher nearthe outer bank, just downstream from the meander apex (Hooke and Harvey, 1983; Markham and Thorne, 1992). Themeander mobility and planform change are the result of a complex combination of physical factors (Hooke, 1995a),

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among which there are two major constraints. River bank resistance is an important factor in river migration potenti-ality and is mainly controlled by the composition of the sedimentary material and its vertical and longitudinalvariation (Thorne and Lewin, 1979; see also the study by Hudson and Kessel (2000), on the Mississippi River prior toembankments) and also by the riparian vegetation, the influence of which varies according to its type, height anddensity (Gregory and Gurnell, 1988; Micheli et al., 2004; Thorne, 1990; . . .). Even if external constraints on meanderbehaviour may be identified, alternative interpretations hypothesize that the different phases of change (from initiationto cutoff ) could be an inherent trend of meanders (Hooke, 2004; Hooke and Redmont, 1992; Stolum, 1996): the riversinuosity increasing to a maximum, critical, state at which meander cutoffs occur.

This study investigates the Beni River, an Amazonian free meandering river, with regards to its erosion–sedimentationrates, together with the subsequent infilling of ox-bow lakes formed by meander cutoffs. The aim of the present workis to further understand the fluvial dynamics of a large southern tropical river and, specifically, to contribute to theknowledge of sediment exportation from the Andes to the Amazonian floodplain. As the source of many of theimportant rivers of the Amazon basin, the Andean Cordillera and piedmont significantly influence the river system anddynamics. The Beni River, whose upper sub-catchments drain the Andean and sub-Andean ranges, is a major tributaryof the Madeira River for water and sediment supply (Guyot, 1993). Mertes et al. (1996) point out the impact of theRio Madeira on the longitudinal readjustment of Solimões–Amazon River fluvial landforms. Mainly because of animportant sand load, the Madeira River enhances the rate of channel change of the Amazon main channel (calculatedon the basis of eroded and deposited areas, Mertes et al., 1996).

Considering that the dominant controls of channel form adjustment are discharge and sediment load (Knighton,1998; Schumm, 1977), our objectives are (i) to determine the meander deformation, accretion and erosion rhythm inrelation to the hydrological functioning and (ii) to build the first model of the contemporary evolution of this BolivianAmazonian floodplain.

The analysis of fluvial adjustment with regard to hydrological and sedimentary processes will be carried out atdifferent temporal and spatial scales. The first step of our approach consists of quantifying of the inter-annual defor-mation of the meanders by calculating deposition and erosion areas in the active channel for the 1996–2001 period.This evaluation concerns the 250 km of our study zone. The first part of the study was undertaken with the aim oflinking erosion–deposition processes with hydrological events. The second part of the study, focusing on the aban-doned meanders, analyses cutoff processes and the post-abandonment evolution over the 1967–2001 period. Thisapproach underlines the strong influence of the main channel’s behaviour on the infilling of the abandoned meandersand allows us to suggest a model describing the present floodplain evolution.

Study Area

The Bolivian Amazonian basin is made up of the Madeira River’s upper basin, occupying two-thirds of Bolivia(Figure 1). The Madeira basin (851 000 km2, 23 per cent of the whole Amazon watershed) is the most importantsouthern Amazon tributary, comprising 15 per cent of its discharge, and 35 per cent of Andean contributions (Guyot,1993; Guyot et al., 1999; Roche and Fernandez, 1988). The Rio Beni catchment (282 000 km2, Table I) is representa-tive of the whole system. The Beni River basin presents highly contrasted climatic and morphodynamic features, as itselevation ranges from 6400 to 200 m.

At Rurrenabaque (Figure 1), the abrupt contact between the piedmont and the floodplain of the foreland basinprovokes a strong decrease of longitudinal gradient and a change of the fluvial pattern to highly mobile meanders. The‘Llanos de Mojos’ is a huge flat plain (mean slope of 10−4 m m−1) of late-Miocene and Quaternary deposits. TheLlanos are inundated for approximately five months extensively (over areas reaching 100 000 km2 and, exceptionally,

Table I. General characteristics of the Rio Beni (HyBAm data, Guyot, 1993; Guyot et al., 1999)

Gauging Basin Elevation Mean discharge Qspea Qsed

b Qdissc

station area (km2) (m) (m3 s−−−−−1) (l s−−−−−1 km−−−−−2) (106 tonnes yr−−−−−1) (106 tonnes yr−−−−−1)

Angosto del Bala 67 500 280 2050 30 192 5·2Portachuelo 119 000 130 2870 24 100 7·8Cachuela Esperanza 282 500 120 8920 32 261 21(Beni + Madre de Dios)

a Specific discharge. b Suspended load. c Dissolved load.



150 000 km2) when abundant precipitation occurs (Denevan, 1980; Loubens et al., 1992). The Beni River floodplaincurrently occupies the western part of the Llanos, because of its counter-clockwise migration under neotectonic effectsof the Bolivian foreland basin (Dumont, 1996; Dumont and Hannagarth, 1993; Plafker, 1964). The Rio Beni basinextends over 800 km of latitude and the present study has targeted 250 km of the river in the Amazonian floodplain,just downstream of the piedmont (Rurrenabaque) – a section where the river develops mobile meanders – to theconfined meanders incised in the Tertiary deposits, downstream of Puerto Cavinas (Figure 1).

Hydrologic and Morphodynamic Features

Thanks to a Bolivian–French partnership, the Rio Beni’s daily discharges have been gauged at several stations since1967 (Figure 1, Table I). One gauging site is located at the Andes–Llanos transition (Angosto del Bala, located 20 kmupstream of Rurrenabaque). The Rio Beni regime may be defined as being of the Austral tropical pluvial type (Guyot,1993; Pardé, 1968) (Table I and Figure 2). In this southern part of the Amazonian basin, the rainy season is fromOctober to April, with December, January and February contributing 50 per cent of the annual rainfall (Hannagarth,1993; Molion, 1993; Ronchail et al., 2002). Therefore, high water levels can be observed during the warm season,from December to April (Figure 2), with January–March representing half of the total water volume. We determinedthe bank-full discharge to be 7000 m3 s−1. Flood maxima show a striking temporal variability, and 11 major floodswere recorded during 1967–2003; the largest discharge reached 20 000 m3 s−1 in March 1999 (Table II).

Figure 1. Situation map – Beni river catchments.

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Several studies showed that flooding is a result of two processes (exogenous and endogenous to the floodplain), andtheir conjunction generates the biggest floods in the Bolivian Amazonia (Bourges and Hoorelbecke, 1995; Bourgeset al., 1992; Bourrel et al., 1999; Pouilly et al., 2004; Roche and Fernandez, 1988; Ronchail et al., 2005). Theexogenous process is defined by the arrival of the flood wave from the upper Beni River. Such a flood is characterizedby neutral white water generated by highly turbid Andean rivers and it spreads through the floodplain via the aban-doned channels. The endogenous process is due to a heavy supply of the local rainwater as well as to the overflow ofgroundwater in the plain. The local tributaries, draining the floodplain, are characterized by acid black water with littlesuspended load (Denevan, 1980; Junk, 1997; Sioli, 1984).

The Beni River, with 192 million tonnes of suspended load per year at Rurrenabaque (Table I), is a white-waterriver. The river represents for this reason a major contribution to the Rio Madeira in terms of sediment yield,supplying 70 per cent of the sediment load (Guyot, 1993). The sediment fluxes show a pronounced seasonal and inter-annual variability: the three months of high discharge represent 82 per cent of the total sediment exportation, andsome years this figure may reach 90 per cent (1968, 1971, 1984, 1999).

In the lowlands, the longitudinal gradient of the river varies from 10−3 to 10−4 m m−1. Over a distance of approxi-mately 250 km, the Beni River develops highly mobile meanders in the floodplain, the sinuosity index ranging from1·6 to 2·5 from Rurrenabaque to the Madidi River junction (Figures 1 and 3). This high index shows a clear relationbetween the fluvial land forms and the meandering fluvial pattern (Brice, 1974; Leopold and Wolman, 1957). Therapid migration of the meander belt forms numerous abandoned channels, forming ox-bow lakes (called ‘lagunas’);27 cutoff processes occurred during the 1967–2001 period. The active bed (the channel and its unvegetated bars) is ofan average width of 600 m. Downstream of the Madidi River junction, the migration of these meanders is blocked bythe consolidated Tertiary deposits.

This specific hydrodynamic functioning (specifically the physico-chemical characteristics of the water as well as theactive sedimentation–erosion processes of the river) lies at the origin of the exceptional biodiversity of the Llanosdescribed by Junk (1997), Pouilly et al. (2004), Salo et al. (1986) and Sioli (1984), among others.

Figure 2. Beni river – mean annual water and sediment discharges (1967–2004, Hybam data).

Table II. Major floods of the Rio Beni at Angosto del Bala (1967–2001, HyBAm data)

Maximum discharge (m3 s−−−−−1)

1968 (25 February) 17 4901971 (27 February) 17 3101972 (22 January) 14 8501972 (20 December) 12 7401974 (16 January) 13 2901978 (5 February) 19 7301982 (6 March) 16 7201983 (18 February) 12 7001984 (2 March) 13 1001999 (20 March) 20 0102001 (13 January) 15 800



Methodology

This study makes use of two related methods: (i) a spatial and temporal analysis of the fluvial land forms; (ii) fieldtopographic and bathymetric surveys.

(i) Analysis of aerial photographs (Corona satellite, 1967) and 17 satellite images (Landsat MSS, 4, 5 and 7: 1975 –upper zone, 1987, 1993 and for every year between 1996 and 2001) allowed us to investigate the temporal evolutionof Beni River’s fluvial land forms, making it possible to characterize the change at two timescales: for 24 years(1967–2001) and at an annual timescale (1996–2001). Concerning the annual timescale approach, the aerialphotographs and satellite images were chosen during the low-water-level period. This gives precise information asto the effect of the annual high water and floods on the changes in the fluvial forms. All data were integrated in aGIS (ArcGis). Our approach is based on the study of various morphodynamic variables in both a synchronic anddiachronic manner through the use of photographs and satellites images. The first approach is the delineation of

Figure 3. 1975–2001 migration of the meandering bend (middle section).

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the active channel (composed by the channel and the unvegetated bars) (Figure 4). The determination of the activebend gives us information on the spatial and temporal variations of channel width and an overlay analysis permitsus to calculate the eroded surfaces (on the concave banks) and the sediment areas (on the point bars). The quantifica-tion of the eroded and sediment areas is similar to that conducted by Mertes et al. (1996) on the Silomões–Amazon River. Extraction of the channel central axis allows calculation of the sinuosity index and of the channelmigration, on the basis of the methodology developed by Hooke and Harvey (1983), or Hooke (1984). Using theimages, the river was discretized into 25, 10 km segments. Second, the delineation of the abandoned channels (ox-bow lakes) was also realized, with regards to their surface, inlet and outlet evolution (alluvial plug) (Figure 4).More precisely, our study targets a quantification of the sedimentation and erosion areas.

(ii) This first analysis was completed in the field by fine-scale measurements:

• the longitudinal gradient of the river was measured with a DGPS;

• the eroded banks, the sedimentation zones (point bars, alluvial plugs) and connection between oxbow lakes andmain channels were verified in the field.

Results

Characterization of erosion and deposition variability in the active channelStrong spatial and inter-annual variability. The average annual migration of the meanders is roughly 30 m per year.This is characterized by strong spatial and temporal irregularities with local maxima of 120–140 m. The calculation ofthe eroded and the sediment areas at the annual time scale (1996–2001) informs us precisely as to the deformation ofthe meanders (Figure (5a)). A first observation is the high mobility of the Beni River: the retreat of the concave bankis 0·07–0·1 km2 per linear km per year, whereas the accretion observed on the point bars is 0·03–0·06 km2 per linearkm per year. In general, the erosion rates recorded are higher than the deposited areas. This result does not imply thatthe volume of sediment loss exceeds that of the volume of the sedimentation. It highlights (i) the important sedimentsupply in the fluvial system by lateral erosion and (ii) the trapping effect of the floodplain (we will discuss this pointin the second part of this study).

Specifically, Figure 5(a) shows a marked spatial distribution along the river of sedimentation and erosion areas inthe active bend. Three main sections are apparent.

• The 90 km upper section (units 1–9) represents the most mobile part of the river. The annual bank retreat can reach0·25–0·27 km2 per linear km (units 5 and 6 in 2000–2001), and the deposition can represent a surface of 0·17–0·2 km2 per linear km.

Figure 4. Morphodynamical variables.



• The erosion–deposition processes are less active in the middle section (units 10–21). The mean annual bank retreatis 0·05–0·1 km2 per linear km, and the accretion on the point bars varies around 0·05 km2, except in local zoneswhere the deposition exceeds 0·1 km2.

• The lateral migration decreases strongly in the lower part, mainly because of the entrenchment in the consolidateddeposits.

When we calculated the annual surfaces, we found visible temporal variations. As can be seen in Table III, annualerosion varies greatly: concave bank retreat was particularly marked during the summers 1998–1999 and 2000–2001,

Figure 5. Interannual variability of erosion and sedimentation in the meandering bend (1996–2001): (a) longitudinal distributionof erosion (grey) and sedimentation area (black); (b) erosion/sedimentation rate.

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exceeding 20 km2 for the whole study section, whereas the 1997–1998 summer floods have provoked a moderate bankretreat, 1·7 times lower. Furthermore, in the meandering bend deposition areas were 2·7 times the extent of theprevious year.

The erosion/sedimentation area rate (‘ESR’, Figure 5(b), Table III) highlights three types of year:

• equivalence in eroded and sedimented surfaces (1999–2000);

• prevalence of deposition areas (1997–1998);

• excess of eroded surfaces over deposition areas (1996–1997, 1998–1999 and 2000–2001).

Detailed analysis of the ESR provides information on its spatial distribution (Figure 5(b)). Concerning the first case(1999–2000, ESR = 0·97), the upper 150 km show an important spatial heterogeneity. Three zones of marked sedi-mentation in the active channel (units 3, 8 and 13–14) are separated by zones of strong bank erosion (units 5–6),which decrease progressively downstream. Downstream, the intensity of erosion and deposition decreases, and spatialvariations are less pronounced. For the 1997–1998 period, the spatial distribution of the ESR is different, undergoingslight fluctuations. An important zone of sedimentation can be seen on the upper 90 km, especially on unit 3. Thesedimentation decreases progressively downstream, except on unit 13.

Concerning the three years of prevailing erosion, significant differences in the spatial distribution of the ESR areobserved. A marked loss of surface is apparent for the years 1996 and 1997 (ESR = 1·5), on the concave banks in theupper 90 km, with a maximum on unit 3. While erosion decreases downstream, it remains predominant in the middlesection, and the rate fluctuates slightly around 1 over the lower 100 km. In contract, for 1998–1999 (ESR = 2·66), therate is hardly beyond 1 upstream, because of an important extension of point bars, whereas erosion occurs mainly inthe middle and the downstream sections (units 8–25). Finally, in the case of the 2000–2001 annual evolution, theproportion of eroded surfaces exceeds greatly the development of point bars (ESR = 3·3), especially in the first 50 km,where the erosion exceeds sedimentation tenfold (when measured in areas). No important zone of sedimentation isobserved.

This first analysis highlights three facts: (i) strong inter-annual variability; (ii) great mobility of the upper 150 km,whereas the 100 lower km are less active; (iii) in this upper zone, unit 3 seems to be particularly mobile and units 13–15 represent a transitional zone.

The links with the hydrological functioning. We wished to highlight the relations between hydrological and geomor-phological dynamics. We therefore tested a possible correlation between two of the most representative parameters ofhydrologic dynamics: the intensity of flood and the duration of discharge close to the bank-full stage discharge orhigher (calculated from daily hydrograms obtained from the Angosto del Bala gauging site). Over the period covered byour study, only the events for the years 1998–1999 and 2000–2001 may be considered extreme (i.e. with a rarer occur-rence), whereas all the other years experienced events which may be referred to as frequent (i.e. bank-full stage dischargeand annual flooding, Table III). Erosion was particularly intense for two hydrological years: 1998–1999 and 2000–2001. Two years correspond to the highest flood peak over the study period: 20 010 m3 s−1 in March 1999 (Figure 6(a))and 15 800 m3 s−1 in January 2001 (Figure 6(b)). The three other years are characterized by low intensity floods.

The bank-full discharge is estimated to be 7000 m3 s−1. Thus we were able to calculate the number of days ofoverflow. The mean duration of the bank-full discharge is 10 days per year (1967–2002), but it greatly varies: 2000–2001 with 24 days, 1999 with 22 days, and the others years registered average values (Table III).

Even if only five years of annual data are available, the linear function shows a good correlation between the ESRand the duration of the bank-full discharge (Figure 7); in other words, the longer discharge exceeds the bank-fullstage, the higher the mean lateral erosion seems to be. Thus, it can be deduced that a long duration of bank-full stagefavours sediment exportation by lateral erosion; furthermore, the modest extension of deposition in the active bedduring the two major floods proves the remobilization of sediment stored in the channel.

Table III. Calculation of annual eroded and sedimented areas in the active channel

1996–1997 1997–1998 1998–1999 1999–2000 2000–2001

Eroded areas (km2) 18·92 14·25 22·93 15·43 24·62Sedimented areas (km2) 13·23 19·71 8·62 15·87 7·4Erosion/sedimentation rate 1·43 0·72 2·66 0·97 3·33Duration of discharge > 7000 m3 s−1 (days) 9 9 22 11 24Flood peak (m3 s−−−−−1) 9476 11 450 20 010 9644 15 800



Concerning the two years of dominant erosion, a hypothesis as to the date of the discharge peak can be proposed. Inthe case of the 2000–2001 flood, the peak discharge occurred at the beginning of the high water level: the maximumwas followed by three phases of high discharge between January and the end of March separated by a short butmarked lowering of the water level (Figure 6(b)). This temporal distribution and the water level fluctuation, bydestabilizing the banks, may explain the marked bank retreat observed all along the study reach and the lack ofdeposition in the meander bend. In the case of the 1998–1999 event, the flood peak occurred at the end of the highwater level period and the discharge decreases rapidly after the maximum (Figure 6(a)). This ‘late’ peak injected in thesystem a large load of sediment (sedimentation zone between units 2 and 7, Figure 5(a)), that probably could not havebeen evacuated downstream because of the rapidly decreasing discharge.

In contrast, brief bank-full stage discharges introduce a spatial heterogeneity with very localized processes of bankerosion and deposition. However, the river response is more chaotic than in the case of a long duration of highdischarge. In some cases, the sediment load coming from the Andes is stored in the upper part of the river (1997–1998), or in local deposition zones alternating with erosion areas (1999–2000).

The slight differences in the ESR observed between years showing similar hydrology are also associated with thesediment load variations. During the great flood of March 1999 a huge sediment load was introduced into the system(Figure 8), much higher than during the January 2001 flood. Significant differences can also be found between 1996–1997 and the following year: the first one is characterized by a larger sediment input that had been partially trapped inthe upstream section.

Influence of slope. As for all Amazonian rivers, the gradient of the Beni River valley is gentle; however, longitudinalnon-negligible variations may be observed and linked with the mobility of the fluvial forms (Figure 9). The first 30 kmare characterized by a rapid decrease of the longitudinal gradient, from 1 × 10−3 to 1 × 10−4 m m−1; and the transitionobserved for unit 3 corresponds to a marked change of the slope. Indeed, the section between units 3 and 9 undergoesan important slope reduction, that could be interpreted as an accumulation area, and the rapid sedimentation–erosionprocesses can be clearly related to the aggradation. Units 9–10 also appear to be an important transition zone, locallymarked by an increase of the slope, preceding a long section of very gentle gradient, characterized, as seen before,by reduced processes of erosion and sedimentation. Downstream of the Rio Madidi junction, the entrenchmentin the consolidated deposits induces weaker slopes, both factors (slope and consolidated deposits) limiting the lateralmigration.

Figure 6. Hydrology of two years characterized by dominant bank erosion. (a) 1998–1999; (b) 2000–2001 (Hybam data).

Figure 7. Relations between the erosion/sedimentation rate and duration of discharge higher than the bank-full level.

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The rhythm of evolution of the ox-bow lakesNumerous abandoned meanders (roughly 160) are spread over the floodplain. The diachronic study of the satelliteimages allows the identification of three generations of abandoned channels in the present floodplain: (i) the ‘ancient’ox-bow lakes formed before 1967; (ii) those created between 1967 and 1987 and (iii) the recently abandoned mean-ders. The calculation of their area at different dates together with the determination of their location relative to theactive channel gives us precise information as to the rhythm of their evolution and associated control factors.

Figure 8. Sediment load of the Beni River (1996–2001; Hybam data).

Figure 9. Longitudinal gradient of the Beni River.



Spatial and temporal distribution of meander cut-offs. The meander cut-off mechanisms depend on the meandermaturity, which is strongly correlated to the migration rate. Concerning the formation of lakes since 1967, a specificspatial distribution can be underlined (Figure 10). Between 1967 and 1987, 10 meander cut-off processes areobserved. Most of them are located in the middle and lower section of the study area. Only two cut-offs were locatedin the upper 100 km. In contrast, since 1988, 16 meander cut-off events occurred and most of them (12) wereproduced in the upper section. All changes are neck cut-offs, except one, which is a chute cut-off.

Figure 10 clearly highlights clusters of cut-offs: for the 1967–1987 period a cluster is located near the small tributaryconfluence (Rio Negro). During the 1987–2001 period, three associations of four or five cut-offs, and one of two cut-offs, can be seen. Only two isolated events occurred during the first period. The avalanche effect (Stolum, 1996;Hooke, 2004) is clearly a control factor: a first meander cut-off creating a local slope increase triggers several cut-offs

Figure 10. Cutoff spatial distribution (the precise date of cutoff can be determined for the 1996–2001 period; they arenoticeable on the left side of the river).



Figure 12. Annual sedimentation area versus distance between the main channel and the ox-bow lake (1996–2001).

Figure 11. 1987–2001 area evolution of the ox-bow lakes (2001 area/1987 area).

in its vicinity with a reduced lag time. This study fits with the simulation developed by Stolum (1996), whichdemonstrated that each cut-off increases the cut-off probability in its vicinity by accelerating local change.

Because of the lack of information on fluvial land forms before 1967, it is not possible to determine precisely theentire cycle of meandering on the Beni River: initialization, growth and cut-off. However, this study reinforces theobservations made in the first part of the paper: two sections of the freely meandering Rio Beni evolve at differingrhythms, the upstream part being the most active zone.

Sedimentation rhythm and evolution schemes of the abandoned channels. We studied the evolution of 160 lagunas onthe present floodplain over the 1987–2001 period (the quality of the 1967 image was not sufficient to calculate thelaguna areas precisely, but it allowed us to determine the location of the ox-bow lakes). Ox-bow lake deposition isrelatively reduced, representing 21 km2, i.e. an average of about 1·5 km2 area reduction per year. With regards to theevolution of the lake areas, Figure 11 shows three unequal ‘populations’. The great majority of the abandoned meandersremains stable; more than the half of the lagunas lost less than 5 per cent of their original area (group A). In contrast, asmall number of ox-bow lakes (10 per cent, group B) disappeared during this period, completely plugged by sediment. Athird group is made up of a relatively small number of lakes (25 per cent, group C) undergoing a certain reduction in size.

A precise annual quantification of deposits between 1996 and 2001 reveals large variations: the 1997–1998 depositsrepresented a maximum of 7 km2, 0·5 km2 and 2·5 km2 during the large floods of 1998–1999 and 2000–2001 respec-tively (Table IV). The annual total deposits are distributed between a small number of lakes: for example, in 1997–1998 an abandoned channel trapped 3·6 km2 (Figure 12)!

Table IV. Calculation of annual deposits in the ox-bow lakes

1996–1997 1997–1998 1998–1999 1999–2000 2000–2001

Sedimented areas (km2) 3·002 7·066 0·548 1·323 2·517Duration of discharge >>>>> 7000 m3 s−−−−−1 (days) 9 9 22 11 24



When analysing the ox-bow lakes subjected to deposits, a specific spatial distribution is revealed: the greatmajority of these lagunas are located near the active channel, at a distance not exceeding 1500 m (Figure 12).The sediment exceptionally ‘travels’ longer distances; only one ox-bow lake lying over 2 km from the activebend showed significant deposition. In other words, this means that only 8–10 per cent of the floodplain widthis subjected to active deposition. This observation does not mean that all ox-bow lakes located in the 1500 m widebend would evolve rapidly; most of them remain stable without significant change in their area. The presence of aconnecting channel between the main channel and the ox-bow lake seems to have no influence. The most surprisingresult lies in the precise location of the lagunas subjected to deposits. All of them are located on the side of a concavebank.

In fact, the three populations identified among the abandoned channels reveal three evolutionary phases. Figure 13helps us understand the post-abandonment evolution. In the case of the Beni River, the evolution of the ox-bow lakeshappens in two phases of unequal infilling separated by a relatively long period of stability. The first phase consists ofthe rapid formation of the alluvial plug. The abandoned meander loses an average of about 8–15 per cent of its areaduring the first 1–3 years; this evolutionary phase corresponds to group C. ‘Laguna Pinky’ is a good example of thisfirst phase: it was created in 1997 and lost 8 per cent of its area in 1998. A thick alluvial plug is built rapidly; thevertical accretion on it is accelerated by vegetation growth, which in this environment is particularly dynamic. Fieldsurveys reveals that the alluvial plugs have an average height of about 2 m immediately after the cut-off and almost

Figure 13. Evolution model of the ox-bow lakes.



reach the height of the floodplain 3 years after. A small connexion channel in the inlet (laguna 148 ‘Salina’) or in theoutlet, or in both parts (laguna Pinky), persists during the first years. It favours the sediment advection, but it alsopermits the evacuation of a non-negligible proportion of the sediment load when the water level decreases in the mainchannel.

After this first short phase of accretion, the sedimentation rate decreases rapidly, as can be seen on laguna Pinky andon laguna Salina (Figure 13). The rapid migration of the active bed toward the opposite side and the progressiveconstruction of the point bar will preserve the ox-bow from active sedimentation. The small connecting channel losesits efficiency in a few years, the increasing distance between the ox-bow and the main channel reducing its slope.The ox-bow lake becomes isolated and evolves very slowly, the sedimentation being dominated by organic deposits(group A). At this scale of spatial and temporal observation, there are no visible significant changes. Laguna 8illustrates this isolation: its size is remarkably stable for at least 12 years (1975–1987). The same stability can beobserved on laguna Salina after 1996 (Figure 13). Therefore, the size of the ox-bow lakes cannot be taken to beevidence of their age.

Lake 8 is also a good example of the second accretion phase (Figure 13), which causes the rapid and often completeinfilling (group B). The returning meander will easily re-incise the former alluvial plug, reoccupying a portion of theabandoned channel, flowing in the opposite direction to before. The adjacent trough will thus be completely filled withsediment in 1 or 2 years. The huge depositional areas – exceeding 0·5 km2 per lake – observed in Figure 12 clearlyshow this third phase of evolution. All ox-bow lakes undergoing a rapid infilling are located in the vicinity of aneroding concave bank. This discontinuous functioning of the abandoned channels is therefore deeply dependent on themigration rate of the meander bend.

Interrelations between cut-offs, ox-bow lake deposits and hydrologic events. First of all, we notice no link betweenmeander cut-off events and the intensity of flooding. In spite of a high frequency of important floods during the1967–1987 period (Table II), the Beni River underwent few cut-off processes. The two large floods (1999 and2001) experienced only one cut-off each (Figure 10), and the ‘modest’ discharge of 1997–1998 caused three cut-offs and one avulsion. These observations corroborate the hypothesis of relative independence between meanderbehaviour and hydrologic events. Hooke and Redmont (1992) and Hooke (2004) on ‘natural’ rivers, and Stolum(1996) on the basis of a simulation model, demonstrated the inherent non-linear behaviour of a meandering system,alternating between chaotic and ordered states. Once the critical state in the form of the meander is reached, clustersof cut-offs occur independently of the intensity of the flood. Thanks to precise chronological and morphologicalevidence, it is possible to suggest that a critical state was reached in 1997–1998 on the upper section of the BeniRiver.

Second, this study forces us to re-examine the links between flood intensity and the evolution of abandonedchannels, highlighting the predominant solidarity between the active meandering channel and the abandoned branches.In the case of the Beni River, the evolution of the ox-bow lakes seems to be more dependent on their position relativeto the active belt than to the flood intensity. Furthermore, when considering the global functioning of the entirefloodplain, this study suggests that large floods do not necessarily imply the presence of important deposits on thefloodplain. The same observation can be made concerning the abandoned channels as for the active bend. The modestflood peaks and the short duration of the efficient discharge seem rather to favour the sedimentation in the ox-bowlakes. Once again, the 1997–1998 year seems to represent an important phase in the behaviour of the meanders andfloodplain.

Two other hypotheses may be put forward to explain (i) the small number of abandoned channels that are subjectedto deposition and (ii) the reduced width of deposition on the floodplain. First, vegetation plays an important part. Atthe end of summer, the water level decreases rapidly, allowing growth of vegetation on dewatered bars and alluvialplugs: vegetation density on the recently abandoned units is well known. This association between vertical accretionand vegetation growth was precisely described by Hooke (1995b) on the Bollin and Dane Rivers. Therefore, thetrapping effect of the dense vegetation is particularly efficient. Second, the advection of the white waters coming fromthe Andes and the sediment diffusion in the floodplain depend on the relative water height between the main channeland the groundwater table. We can suppose that a high water level in the floodplain and in the lagunas will limit thesediment advection, whereas a modest flood (such as the one that occurred during the summer of 1998) occurringwhen the floodplain is relatively dry will easily progress in the abandoned channel network.

Discussion

The efficiency of bank-full stage discharges has been demonstrated in previous works. Because of their frequency,they regenerate the existing fluvial forms and pioneer vegetation in the active channels (Dury, 1961; Williams, 1978).



In the case of the Rio Mamoré in the Bolivian Amazonia (Bourrel and Pouilly, 2004; Charriere et al., 2004) similarobservations have been made. Discharges lower than the overflow level creates rapid bank retreat; nevertheless, theRio Mamoré experienced a longer duration of discharges close to bank-full and weaker inter-annual hydrologicvariability. This also agrees with Harvey’s observations (1975) that show that the threshold-pool space (depending onbed load remobilization) reveals a better correlation with more frequent discharges than the bank-full stage value, thanwith rarer events. In the case of the Beni River, the flood intensity is the main factor controlling bank erosion.However the bank-full level duration also seems to have a determining morphogenic effect, by transferring thesediment load supplied by the massive erosion in the Andes.

When comparing the Beni River with other Amazonian fluvial systems, peculiar features can be pointed out –particularly their mobility. On the Solimões–Amazon River, the average rate of migration is 25 m yr−1 over a period of120 years, with maxima of about 140 m yr−1 (Mertes et al., 1996); Kalliola et al. (1992) calculated the rapid bankretreat on the same river as reaching 200–400 m yr−1 near the Peruvian border. According to the authors, the migrationrepresents an average of about 0·5 per cent of the Solimões–Amazon main channel width and a maximum of 3 percent. Salo et al. (1986) measured average migration rates of 12 m on a Peruvian Amazonian tributary, the Rio Manu(smaller than the Rio Beni). This annual migration represents a regeneration rate of 3·7 per cent of the presentfloodplain, and Salo et al. (1986) emphasize the positive impact of this disturbance on the biological diversity of theforest. Greater instability is to be observed on the Beni River: an average migration of 5–6 per cent of the mainchannel width and a maximum rate of 20–25 per cent. Comparable values are obtained on the Mamoré River, anotherBolivian tributary in the Llanos characterized by mobile meanders (personal observation). The instability of the BeniRiver is at the basis of an exceptional regeneration rate of the floodplain (Figure 14): over the 1987–2001 period, theriver regenerated between 11 per cent and 35 per cent of its floodplain, i.e. 1–2·5 per cent per year, comparable tothose measured on the Manu River (Salo et al., 1986).

Figure 14. Regeneration rate of the Rio Beni floodplain.



Several explanations may be advanced. First, the calculation of specific stream power (or power per unit bed area,Bagnold, 1966) helps understand spatial variations in the whole Amazonian catchment:

ω(W m−2) = ρgQbfSe/w (where Qbf is bankfull discharge, Se longitudinal gradient and w channel width)

In the case of the Rio Beni, the specific stream power varies between 114 and 57 W m−2 in the upper 50 km, and it is22–11 W m−2 in the middle and lower sections. Concerning the Solimões–Amazon River, the specific stream power isaround 10 W m−2 near the Peruvian–Brazilian border. Second, because of its upper location in the Amazon basin,the Beni River is directly influenced by the water and sediment inputs coming from the Andes and sub-Andeanranges. The rapid deformation of the meander bend expresses the river adjustment to the important sedimentload and the marked water discharge variations. The evolution of the abandoned channels is strongly correlatedto the meander belt activity, which plays a major role in sediment sequestration and in the construction processesof the floodplain. In contrast, the Solimões–Amazon fluvial system is located (i) in the Equatorial zone andtherefore depends on a different climato-hydrological functioning and (ii) far away from sediment sources. Itsfluvial pattern is significantly different: the Brazilian river develops an anastomosing pattern made up of a mainchannel – relatively sinuous and unstable in the upper section and more rectilinear and stable downstream – accompa-nied by secondary branches surrounding large islands (Mertes et al., 1996). On the same river between the Purus andthe Negro River confluences, Latrubesse and Franzinelli (2002) have described a low sinuous course, locallyanastomosed. These studies conducted on the Amazon River emphasize the role played by the secondary channelsassociated with levee complexes in the construction of the floodplain: more sinuous and mobile than the main channel,these secondary branches are characterized by rapid sedimentary activity and generate scroll-dominated forms.The creation of crevasse-splays through the levees also favours the sedimentation in old scroll-bars by decanting,especially on the lower Amazon River, where the relatively straight main channel builds a levee that tends to confinethe migration.

In the Beni and Mamoré floodplains, Aalto et al. (2003) analysed the 210Pb activity profiles from sediment coresand, on this basis, suggest that episodic sedimentation is the predominant mechanism for floodplain accumulation andthat two major sedimentary mechanisms control the deposition. (i) In the proximal floodplain (<300 m from the activechannel), frequent decanting of sediment over bank during annual floods creates natural levees. (ii) The distal floodplainundergoes episodic sedimentation: discrete packages of sediment of uniform age, which are 20–80 cm thick (with amaximum of 2 m), are associated with episodic deposition events (every 8 years). The authors interpret these specificdeposits as being probably delta-shaped sediments of small crevasse-splays formed during failure of the levees. Thissedimentation model could correspond to the middle and lower Amazon River model. However, the present multi-scale analysis of the Beni River obtained through the use of satellite images and field survey does not reveal evidencesof levee and crevasse-splay formations. The very rapid migration of the meander bend renders the building of leveesdifficult, which should have confined the channel. Furthermore, our geomorphologic approach pointing out thespecific sedimentation mechanisms in the abandoned channels allows us to build other hypotheses concerning thedepositional process on the Beni River: the ‘episodic deposits’ identified by Aalto et al. (2003) are probably related tothe two sedimentation phases of the ox-bow lakes. The first phase is characterized by the rapid construction of thealluvial plug on the inlet and outlet of the abandoned channel; the water level decreases during the dry season, thushelping the colonization of the pioneer vegetation. Sedimentation to nearly floodplain level is achieved within about3 years at the entrance of the abandoned meanders. The rapidly meandering new channel creates a point bar adjacentto the alluvial plug formed on the upper and lower parts of the abandoned channel, and this first phase is concluded bythe ox-bow lake isolation. This evolution scheme can be considered as classic, even if the activity of the Beni River ismore rapid than for other meandering systems. Several studies conducted on meandering rivers of Great Britain andFrance show that infilling of ox-bow lakes is more progressive and depends mainly on cut-off channel age, frequencyof floods, curvature of the main channel and connectivity with the main channel. On the Dane and the Bollin Rivers,Hooke (1995b) measured a rapid sedimentation during the first year after cut-off, the sedimentation being achievedwithin about 6 years at the entrance to the abandoned channels. Gautier et al. (2001) on the Loire River, also noticehigher accretion rate on the upstream plug of abandoned channels. In the case of the Ain River, Citterio and Piégay(2000) and Piégay et al. (2000) enlighten the role played by the connection: backflow in the outlet connection seemsto favour an active sedimentation with suspended load; whereas an inlet connection limits deposits because of rapiderflows. Furthermore, the ox-bow lakes located on the convex side of active meanders undergo a rapider sedimentation.The specificity of this Bolivian river lies in the third phase, that is marked by a very short – but particularly efficient– accretion (1–2 years), and that plays a major part in the construction of the floodplain. The remnant lake iscompletely deposited when it is located near the concave side of a migrating meander, differentially from ox-bowlakes located in the temperate zone.



Conclusion

This study is a contribution to the knowledge of the Amazonian rivers, where few studies have been conducted onfluvial geomorphology, except for the Solimões–Amazon River. The lack of long-term geomorphologic and dischargedata on the Beni River currently makes its hard to compare more precisely both floodplain systems. However, ouranalysis, conducted over a period of 25 years, allows us to document the specific features of this river in theAmazonian basin. The recycling time of the Beni River floodplain is about 10 times higher than the Solimões–Amazon River, where Mertes et al. (1996) calculated an alluvial plain recycling time ranging from 1000 to 4000years. These results clearly explain the impact of the Madeira River, collecting water and sediment from the BeniRiver, on the Solimões–Amazon River longitudinal readjustment.

This approach also investigates the links between hydrologic functioning and the adjustment of river forms. The bank-full discharge is identified as a major factor of the active channel evolution; its duration determines the exportation ofthe sediment load provided by the Andean and sub-Andean ranges and the regeneration of the alluvial stock of thefloodplain by bank erosion. The functioning of abandoned branches is strongly associated with the great mobility ofthe main channel rather than to flood intensity. Highlighting the absence of a temporal link between the meander cut-offs and the flood events, the Beni River also reinforces the hypothesis of an inherent behaviour of the meandering system.

AcknowledgementsThis research is supported by the ‘Hydrologie du Bassin Amazonien’ (Hybam) program (Institut de Recherche pour le Développement,France, SENAMHI: National Meteorology and Hydrology Department of Bolivia, and Universidad Major San Andres, La Paz), the‘Programme Relief’ (Centre National de la Recherche Scientifique et Institut des Sciences de l’Univers, France) and the Laboratoirede Géographie Physique (CNRS, France).

13 satellite images were provided by ‘Global Landcover Facility’, courtesy of US Geological Survey (http://www.usgs.gov).Thanks are also to the students of Universidad Major San Andres, for their in-the-field help.

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