EXCHANGES OF SEDIMENT BETWEEN THE FLOOD-PLAIN AND CHANNEL OF THE AMAZON RIVER IN BRAZIL

Sediment transport through the Brazilian sector of the Amazon River va lley, a distance of 2010 km, involves exchanges between the channel an d the flood plain that in each direction exceed the annual nw of sedim ent out of the river at Obidos (similar to 1200 Mt yr(-1)). The exchan ges occur through bans: erosion, bar deposition, settling from diffuse overbank flow, and sedimentation in flood-plain channels. We estimate d the magnitude of these exchanges for each of 10 reaches of the valle y, and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and now records to defin e a sediment budget for each reach, Residuals in the sediment budget o f a reach include errors of estimation and erosion or deposition withi n the channel. The annual supply of sediment entering the channel from bank erosion was estimated to average 1570 Mt yr(-1) (1.3 x the Obido s flux) and the amount transferred from channel transport to the bars (380 Mt yr(-1)) and the flood plain (460 Mt yr(-1) in channelized flow ; 1230 Mt yr(-1) in diffuse overbank now) totaled 2070 Mt yr(-1) (1.7 x the Obidos flux). Thus, deposition on the bars and flood plain excee ded bank erosion by 500 Mt yr(-1) over a 10-16 yr period. Sampling and calculation of sediment loads in the channel indicate a net accumulat ion in the valley floor of approximately 200 Mt yr(-1) over 16 yr, cru dely validating the process-based calculations of the sediment budget, which in turn illuminate the physical controls on each exchange proce ss, Another 300-400 Mt yr(-1) are deposited in a delta plain downstrea m of Obidos, The components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain, which In turn are influenced by tectonic features of the Amazon structural trough.