The grain size distribution of river bed sediments in the Rio Madeira drainage basin shows rapid deposition of the coarser material (>10 mm) as the river flows out of the Andean range to enter the Amazon plain (Llanos). In the Andes, the suspended sediment grain size varies between 0.02 and 0.10 mm in diameter, while in the Llanos the suspended load is dominated by fine silts, between 7 and 13 μm in diameter. In the Llanos, neither the bed sediment nor the suspended sediment varied along a 800 km reach.