Les flux de matières dissoutes et particulaires exportés des andes par le rio béni (Amazonie bolivienne), en période de crue

The Béni river, located in the Rio Madeira drainage basin (Figure 1), contributes significantly to the sediment exportation from the Andes to the Amazonian basin. Although most of transport takes place during floods, the nature of the load during high flow was poorly known. To investigate this topic, an annual flood episode (10000 m₃. s_-1) has been sampled near Rurrenabaque, at the ‘Angosto del Bala’ PHICAB gauging station (Figure 1 and Table I) that controls the whole Andean catchment of the Béni river (67500 km₂). This watershed presents highly contrasted geomorphic and climatic features, including some semi-arid sedimentary highly erodible basins, and crystalline, hyperhumid, densely vegetated ones. Altitude is ranging from 6400 to 250 meters.Gauging and sampling (Figures 3 and 4 and Table II) was carried out daily from March 15 to 30, 1988. Major elements (Tables III and V) and organic carbon (Table VII) were analysed in dissolved and suspended solids, clay mineralogy (Table IV) and trace elements (Table VI) in the suspended sediments.Concentrations of Total Dissolved Solids (TDS) and Total Suspended Sediments (TSS) are highly variable (Figure 4) but linked together (Figure 5) as TDS and TSS probably come from the same basins. Inversely, the chemical composition of TDS as well as TSS is relatively stable during the sampled period. Almost 90 % of organic carbon exported during this flood is particulate.Corresponding budgets are calculated. Particulate material accounts for 99 % of the total load. On an annual basis, this high flow episode (4 % of the year time) may contribute to 30 % of the Beni river mean annual sediment exportation to Amazonia.