中文摘要：

通过对长江上游干流和主要支流年输沙量变化特征、降雨条件、20世纪90年代土地利用和水库拦沙变化过程的对比分析研究，揭示出三峡水库蓄水运用前长江上游年输沙量存在1-2个突变点，具有显著的阶段性变化现象。其中嘉陵江流域年输沙量在1985年以后和1994年以后显著减少，水库拦沙是主要原因；岷江流域年输沙量在1967年以后和1994年以后显著减少，降雨变化比水库拦沙的贡献大；乌江和整个上游年输沙量在1985年以后和1992年以后显著减少，也主要与水库拦沙有关。20世纪最后十年间的土地利用变化对上游侵蚀输沙影响不显著。

英文摘要：

The characteristics of changes in annual sediment load recorded at five hydrological stations close to the outlet of the upper Yangtze River and its four main tributaries are statistically analyzed. The changes in annual sediment load are compared with the changes in factors influencing soil erosion and sediment detention by reservoirs given by previous studies for deciphering the contributions to the changes in annual sediment load from precipitation fluctuation, landuse changes, and sediment detention by reservoirs. The results show that one or two abrupt descents or ascents with statistical significance exist in the time series of annual sediment load of the four tributaries and the main stem of the upper Yangtze River. The annual sediment load of the Jialing River descended significantly after 1985 and 1994 by 67.2% and 60.4%, respectively. The two abrupt descents can be attributed to the sediment detention by reservoirs. An obvious decline of annual sediment load occurred at the outlet of the Wujiang River and the whole upper Yangtze River after 1967 and 1992, respectively. It can be also mainly related to the sediment detention by reservoirs. In contrast, the significant descents of annual sediment load of the Minjiang River after 1967 and 1994, respectively, are mainly the results of changes in precipitation, and the effects of dams are proved to be a minor contributor. Although a large scale soil conservation project has been carried out since 1989, this study does not find detectable impacts of landuse change in the last decade of the 20^th century on sediment yield in the upper Yangtze River.