LAUSR

Abstract Multi-annual contemporary flood event overbank sedimentation rates were quantified on the World's third largest river in terms of discharge, the tropical lower Orinoco. We discuss the role of variables at the basin and reach scales that contributed to the complexity of spatio-temporal overbank sediment deposition patterns. Monitored in situ plots were characterized by distance to the main channel, hydroperiod, different geomorphological units, and vegetation cover. Flood event sedimentation rates showed a high spatial variability ranging from the absence of sediment deposition up to 225.46 kg m-2 yr-1. Banks and levees received relatively high amounts of sediment (39.6 kg m-2 yr-1), whereas observed mean sedimentation rates on the more distant floodplain and backswamps tended to be lower (17.7 kg m-2 yr-1). Significant differences in sedimentation rates were observed in two major vegetation types: dense herbaceous and shrubby vegetation (42.2 kg m-2 yr-1) and floodplain forest (12.7 kg m-2 yr-1). However, overbank sedimentation patterns also reflected imbricated hydrosedimentary and biogeomorphological vegetation feedbacks that co-construct fluvial landforms. The incidence of an El Nino–Southern Oscillation–La Nina episode during the study period on sediment availability and floodplain sedimentation suggests that within whitewater rivers, where suspended sediment concentrations are naturally high, hydrological connectivity seems to be more important for floodplain sedimentation than variations in suspended sediment concentrations. These results may provide a good basis for future biogeomorphological investigation projects using complementary methodologies, in order to better anticipate global change and fluctuations in the occurrence, strength or duration of El Nino–La Nina episodes in the tropical zone and their consequences for flood discharge and sediment dynamics during channel–floodplain exchanges.