Sediment dynamics in the lowermost Amazon

The sedimentological character of material in transport and an the riverbed were investigated in order to record the sedimentary dynamics in the lower most Amazon. Suspended sediment concentration measurements show that net de position occurs on the delta plain (between Obidos and the Xingu River), wh ile downstream suspended sediment transport remains approximately constant. Lateral sediment input is compensated by erasion in deeply incised channel s keeping the region in a dynamic equilibrium. The surficial distribution o f bedload sediments carried by the Amazon shows a corridor of sands (channe l deposits) bordered by silty sediments at the margins. It consists chiefly of very fine sands, which become fine and medium-coarse sands laterally (F igure 6). The development of bedforms on the sand surface reflects strong b ottom currents and sediment transport, as well as tidal effects. Nearshore sedimentary facies identified were interlaminated fine and coarse silt, int erlaminated silt and sand, faintly laminated mud, and mottled mud.