Sediment Dynamics in the Lowermost Amazon

Abstract

The sedimentological character of material in transport and on the riverbed were investigated in order to record the sedimentary dynamics in the lowermost Amazon. Suspended sediment concentration measurements show that net deposition occurs on the delta plain (between Obidos and the Xingu River), while downstream suspended sediment transport remains approximately constant. Lateral sediment input is compensated by erosion in deeply incised channels keeping the region in a dynamic equilibrium. The surficial distribution of bedload sediments carried by the Amazon shows a corridor of sands (channel deposits) bordered by silty sediments at the margins. It consists chiefly of very fine sands, which become fine and medium-coarse sands laterally (Figure 6). The development of bedforms on the sand surface reflects strong bottom currents and sediment transport, as well as tidal effects. Nearshore sedimentary facies identified were interlaminated fine and coarse silt, interlaminated silt and sand, faintly laminated mud and mottled mud.