Variations in quantity, composition and grain size of Changjiang sediment discharging into the sea in response to human activities

In order to evaluate the impact of human activities (mainly dam building) on the Changjiang River sediment discharging into the sea, the spatialoral variations in the sediment load of different tributaries of the river were analyzed to reveal the quantity, grain size and composition patterns of the sediment entering the sea. The results show that the timing of reduction in the sediment load of the main stream of the Changjiang was different from those associated with downstream and upstream sections, indicating the influences of the sub-catchments. Four stepwise reduction periods were identified, i.e., 1956-1969, 1970-1985, 1986-2002, and 2003-2010. The proportion of the sediment load originating from the Jinsha River continuously increased before 2003; after 2003, channel erosion in the main stream provided a major source of the sediment discharging into the sea. In addition, in response to dam construction, although mean grain size of the suspended sediment entering the sea did not change greatly with these different periods, the inter-annual variability for sediment composition or the relative contributions from the various tributaries changed considerably. Before 2003, the clay, silt and sand fractions of the river load were supplied directly by the upstream parts of the Changjiang; after 2003, although the clay component may still be originating mainly from the upstream areas, the source of the silt and sand components have been shifted to a large extent to the river bed erosion of the middle reach of the river. These observations imply that the load, grain size and sediment composition deposited over the coastal and shelf water adjacent to the river mouth may have changed rapidly recently, in response to the catchment changes.