Sediment Yield in Major Sediment Source Areas of the Upper Changjiang River Basin in Response to Human Activities

The drainage areas between Shigu and Pingshan(except the Yalongjiang River basin) of the Jinshajiang River and the Jianglingjiang River basin are major sediment source areas of the upper Changjiang River.From 1954 to 1975, suspended sediment load and concentration at Pingshan station on the Jingshajiang River showed no trending variation,but afterwards they tended to increase.From 1954 to the early 1980s,suspended sediment load and concentration at Beibei station on the Jialingjiang River showed no trending variation,but after 1983 they tended to decrease.From 1954 to the mid-1980s,suspended sediment load and concentration at Yichang station on the Changjiang River showed no trending variation,but after 1985 they tended to decrease.A analysis of relationship between sediment load and river flow indicates that,on condition that annual river flows are equal,the annual suspended load at Pingshan station during the period 1976-1996 was larger than that during the period 1954-1975,due to large-scale mining and construction of iron-steel industry and communication lines,which increased soil loss.On contrast,due to reservoir construction,land terracing,tree planting and natural restoration of vegetation,sediment yield was reduced,and then the annual suspended sediment load at Beibei station on the Jialingjiang River during the period 1983-1996 became much smaller than that during the period 1954-1982.The annual suspended sediment load at Yichang station on the Changjiang River during the period 1985-1996 was also smaller than that during the period 1954-1996.It has been found that there is a threshold in annual river flow,at which the sediment reduction by soil-water conservation measures equals 0.For Beibei station and Yichang station,this threshold is 114 billion m3 and 480 billion m3,respectively.Due to differences in natural conditions and human activities in the two major sediment source areas,the lower Jinshajiang River basin and the Jialingjiang River basin,the response of sediment yield in the two areas are different,just in reverse directions.From viewpoint of drainage basin system,this reflects some complicated spatial behavior,resulting from different responses in different parts of a huge fluvial system.Study of these complicated spatial behaviors may help to get a better understanding of the mechanism of response of sediment yield to changing environmental factors,which is useful to more effective control of soil loss and sediment-related hazards.