Taking the typical tributaries in the sediment-rich area with more coarse ones inthe middle reaches of the Yellow River as an example, this paper analysed the causeof reduction of incoming sediments in the 1980s. The authors hold that the reductionof rainfall and intensity of rainfall was the main cause of sediment reduction in the1980s, whose effects upon sediment reduction were about 70%, while the effects of thecomprehensive control over the watershed upon the degree of sediment reduction accounted for 30% of total sediment reduction.