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

摘要

In order to evaluate the impact of human activities (mainly dambuilding) on the Changjiang River sediment discharging into the sea, thespatial–temporal variations in the sediment load of different tributaries ofthe river were analyzed to reveal the quantity, grain size and compositionpatterns of the sediment entering the sea. The results show that the timingof reduction in the sediment load of the main stream of the Changjiang wasdifferent from those associated with downstream and upstream sections,indicating the influences of the sub-catchments. Four stepwise reductionperiods were identified, i.e., 1956–1969, 1970–1985, 1986–2002, and2003–2010. The proportion of the sediment load originating from the JinshaRiver continuously increased before 2003; after 2003, channel erosion in themain stream provided a major source of the sediment discharging into thesea. In addition, in response to dam construction, although mean grain sizeof the suspended sediment entering the sea did not change greatly with thesedifferent periods, the inter-annual variability for sediment composition orthe relative contributions from the various tributaries changedconsiderably. Before 2003, the clay, silt and sand fractions of the riverload were supplied directly by the upstream parts of the Changjiang; after2003, although the clay component may still be originating mainly from theupstream areas, the source of the silt and sand components have been shiftedto 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 compositiondeposited over the coastal and shelf water adjacent to the river mouth mayhave changed rapidly recently, in response to the catchment changes.