本文阐明了大型水库的拦沙和滞洪削减洪峰,降低挟沙能力的减沙机制,给出了大型水库下游河流输沙量时空变化的坝下河段无泥沙补给、少量泥沙补给和有大量泥沙补给三种模式。在此基础上,通过水库修建前后下游河流水沙变化的对比分析,认为大型电站水库的修建是20世纪80年代以来乌江、90年代以来金沙江、嘉陵江输沙减少和70年代以来岷江输沙波动的重要驱动力。宜昌站1992-2002年期间的输沙量比1956-1992年期间减少28.2%,主要归功于嘉陵江来沙减少和1998年以后的金沙江来沙减少;2003-2007年期间的输沙量又比1992-2002年期间减少82.2%,主要归功于三峡水库的蓄水拦沙。

The mechanics of trapping sediments and detaining floods to reduce flood discharges and sediment-carrying capacity by large-scale reservoirs on main streams is illustrated.Three patterns of sediment yield variation with distance and time in the river channels below reservoirs under conditions of little,limited and large sediment recharge from the downstream watershed are shown in this paper.By analyses of runoff and sediment yield changes after construction of large-scale reservoirs on main streams,it is suggested that construction of those reservoirs was an important driving force of sediment yield reductions since 1990s in the Jinsha and Jialing Rivers and since 1980s in the Wujiang River as well as of sediment load fluctuation since 1970s in the Minjiang River.The annual sediment yield during the period of 1992-2002 decreased by 28.2% of the yield during the period of 1956-1992 and the reduction was mainly contributed by the sediment yield reductions of the Jialing and Jinsha Rivers since 1998.The annual sediment yield at Yichang station during the period of 2003-2007 decreased by 82.2% of the yield during the period of 1992-2002 and the rapid reduction since 2003 was no doubt caused by the Three Gorges Reservoir