DISSOLVED AND SUSPENDED SEDIMENT LOADS EX PORTED FROM THE ANDES BY THE BENI RIVER (BOLIVIAN AMAZONIA), DURING A FLOOD

The Beni river, located in the Rio Madeira drainage basin (Figure 1), contributes significantly to the sediment exportation from the Andes t o the Amazonian basin. Although most of transport takes place during f loods, the nature of the load during high flow was poorly known. To in vestigate this topic, an annual flood episode (10000 m3. s-1) has been sampled near Rurrenabaque, at the 'Angosto del Bala' PHICAB gauging s tation (Figure 1 and Table I) that controls the whole Andean catchment of the Beni river (67500 km2). This watershed presents highly contras ted geomorphic and climatic features, including some semi-arid sedimen tary highly erodible basins, and crystalline, hyperhumid, densely vege tated ones. Altitude is ranging from 6400 to 250 meters. Gauging and s ampling (Figures 3 & 4 and Table II) was carried out daily from March 15 to 30, 1988. Major elements (Tables III & V) and organic carbon (Ta ble VII) were analysed in dissolved and suspended solids, clay mineral ogy (Table IV) and trace elements (Table VI) in the suspended sediment s. Concentrations of Total Dissolved Solids (TDS) and Total Suspended Sediments (TSS) are highly variable (Figure 4) but linked together (Fi gure 5) as TDS and TSS probably come from the same basins. Inversely, the chemical composition of TDS as well as TSS is relatively stable du ring the sampled period. Almost 90 % of organic carbon exported during this flood is particulate. Corresponding budgets are calculated. Part iculate material accounts for 99 % of the total load. On an annual bas is, this high flow episode (4 % of the year time) may contribute to 30 % of the Beni fiver mean annual sediment exportation to Amazonia.