Variation in grain size of suspended load in upper Changjiang River and its tributaries by human activities

Based on data from 5 hydrometric stations, Pingshan station on the Jinshajiang River, Gaochang station on the Minjiang River, Wulong station on the Wujiang River, Wusheng station on the Jialingjiang River and Yichang station on the Changjiang River, a study on the temporal variation in grain size of suspended sediment load in the upper Changjiang River has been made. The results show that in the past 40 years, grain size of suspended sediment load of main stem and major tributaries of the upper Changjiang River has a decreasing trend. This decreasing trend can be explained by the effect of reservoir construction and implementation of soil_water conservation measures. The reservoirs in the upper Changjiang River basin, all used for water storage for hydro_electric generation and/or irrigation, trap coarse sediment from drainage area above the dam, and thus, the released sediment becomes much finer than before the construction of reservoirs. The downstream channels are all gravel_bedded or even in bedrock, consisting of little fine sediment, and thus, the released flow can hardly get supply of fine sediment through eroding the bed. Then, after the downstream adjustment, the grain size of suspended sediment is still fine. Large_scale soil_water conservation measures have significantly reduced sediment yield in some major sediment source areas. The relative coarse sediment is trapped, and thus, the sediment delivered to the river becomes finer. A multiple regression analysis has been conducted to determine the influence of sediment fining and sediment yield variation from different water_sediment source areas. Using the two multiple regression equations, the variation in suspended sediment median size entering the middle Yangtze River can be roughly estimated, based on the variations in suspended sediment grain size and sediment yield from different source areas.