Abstract
The upstreamβdownstream sediment budget along the Napo River (100 520 km^2^, 6300 m^3^ s^β1^) was studied in the Andean foothills of Ecuador, at the west of the Amazon basin. A comparative study was made during four hydrological cycles (2001β2005) for three hydrological stations located upstream, and during one hydrological cycle (2004β2005) for the fourth one located near the mouth of the Napo River (region of Iquitos in Peru). This analysis showed an unusual increase in the concentration of suspended sediment recorded for the western part of the Amazon plain. Like the runoff (81 l s^β1^ km^2^), which is a world's maximum, the erosion rate (1160 t km^β2^ year^β1^, i.e. 47% of total suspended solid (TSS) export at the exit of Ecuador), one of the highest for a floodplain basin is the result of a stepper slope than in the rest of the Andean foothills, where typically sedimentation phenomena are predominant, and can be explained in part by a greater tectonic activity. Similar phenomenes were evidenced in small mountainous rivers in New Guinea (Milliman and Syvitski, 1992; Milliman, 1995).
On the headwaters of the Napo River drainage basin, the tectonic uplift causes the Pastaza Megafan's existence. This progressively diverts the course of Napo River towards north and also provokes the remobilization of fine fluvial deposits. Moreover, this geodynamic trend is completed by the impact of volcanic eruption, earthquakes and landslides. The combination of these phenomena, so common in the region, has provided a large sediment transfer, not only at present but also in the past, as can be confirmed by the presence of incised terraces, mainly formed by volcanic materials.
Then, these results were compared with a similar study carried out further south in the Madeira basin at the Bolivian foothills. These studies show the spatioβtemporal variability of the relation between sediment transfer and geodynamic processes at the Andean Piedmont. Copyright Β© 2009 John Wiley & Sons, Ltd.