Abstract:To examine the impacts of the Manwan reservoir sediment trapping on the physical and chemical characteristics of the sediment downstream, laser grain size, XRD and XRF were employed to test the grain sizes, mineralogy and element compositions of suspended sediments and bed loads sampled from the mainstream and many tributaries in the middle and lower parts of the Lancang River. Results indicated that: the grain sizes frequency curves are mostly showed as unimodal and charactered by wide range of grain size distributions; the suspended sediments are well sorted; the bed loads appear bimodal distribution and narrow range of grain sizes, and poorly sorted; suspended sediments and bed loads are primarily composed of quartz which account for 60% to 80%; element compositions of suspended sediments are not different very much between upstream and downstream,but those of bed loads present a decreasing trend from upstream to downstream; sedimentation in the Manwan Reservoir has evident effects on the grain size and mineralogy of sediment, also causes some changes in the element composition, which is showed as normal elements but silicon take little changes, and some heavy metal are trapped in the reservoir.