The effect of the exceptionally low water discharge on sediment characteristics of the middle and lower Yangtze River

Based on meteorologic data, historical sediment data and current & sediment data collected in October 2006 in the Yangtze River Basin, the analyses of monthly mean water and sediment discharge characteristics, meteorologic characteristics and the effect on suspended sediment concentration and suspended sediment grain diameter under the exceptionally low water discharge were carried out. The results showed that the sediment discharge decreased remarkably in the year of low water discharge, and the sediment discharge of flood season at Datong Hydrographic Gauging Station is only 19.8% of the mean value from 1985 to 2000. The mean suspended sediment concentration in October 2006 was 0.057 kg/m³ in the middle and lower Yangtze River, being only 20.6% of the mean value in October from 2003 to 2005. The mean suspended sediment median diameter in October 2006 was 4.8 μm, being only 26.3% of the mean value of historic records and 41.8% of the annual mean value recently. The synthetic analysis of suspended sediment concentration, suspended sediment diameter and scouring/silting characteristics showed that the vertical water and sediment exchange was intense in the reach from Chenglingji to Hukou, where the channel adjustment has taken place frequently in recent years. Under the background of the exceptionally low water discharge and the completion of the Three Gorges Project, the contribution of the suspended sediment concentration from Dongting and Poyang lakes to the main stream was greater than that of any other takes and tributaries, and the contribution of Poyang Lake was greater than that of Dongting Lake. The influence of the suspended sediment diameter from the Hanjiang River and Chaohu Lake to the main stream was greater than that of any other lakes and tributaries, while the influence of Dongting and Poyang lakes was relatively small.