Impacts of cascade reservoirs on the longitudinal variability of fine sediment characteristics: A case study of the Lancang and Nu Rivers

The Lancang River and Nu River are parallel rivers located in southwestern China; while the Lancang River has several dams, the Nu River has none. To explore the impacts of cascade reservoirs on the grain size and mineral composition of fine sediments, 9 fine sediment samples from the Nu River and 29 samples from the Lancang River were collected and tested. The results indicate that in the free-flowing reaches (FFRs) of the Lancang River, the sediment grain sizes fined systematically and the mineral compositions varied slightly, and these results were similar to those of the Nu River without dams along the whole mainstream. In the regulated reaches (RRs) of the Lancang River, the sediment grain sizes and mineral compositions indicated discontinuous fluctuations due to the dams trapping. In the reservoir head areas, the sediments were composed of fine silt or medium silt with 50% clay minerals, but the sediments immediately downstream of the dams were fine sand or medium sand with 71% quartz and feldspar. The relative abundance of each mineral was altered differently by passing through the RRs: quartz, chlorite, and Mite increased by 13%, 33% and 20%, respectively, while feldspar and calcite decreased by 26% and 80%, respectively, and dolomite and pyrite were completely trapped in the cascade reservoirs. All these common minerals may be replenished except pyrite, which originates from mining areas in the Lancang sub-basin.