Sediment delivery to the Three Gorges 2: Local response

Information on soil erosion and sediment yields is very limited in the immediate vicinity of the Three Gorges Project Ž . TGP , although the inceptisols developed on Triassic shales and sandstones in this area are known to be highly susceptible Ž . to erosion. Estimated sediment yields for a small reservoir catchment Yiwanshui in Changshou County, downstream of Ž y2 y1. Chongqing, are high around 3500 t km a and similar to the data from 38 reservoir investigations in Sichuan basin, but exceeding the yields estimated in the feasibility studies for the TGP. The high sediment yield reflects severe soil erosion due to cultivation aligned with slope on steep slopes, which is very common throughout Sichuan Basin and particularly in the TGP area. The total sediment delivered to the Yangtze from the TGP area has been estimated, using small reservoir data and sediment load measured at hydrological stations. These are comparable with previous estimates but there is much temporal variability in sediment supply from different parts of the Three Gorges region, suggesting the importance of local impact on sediment flux. Resettlement of agricultural activity around the Three Gorges Reservoir is likely to further enhance soil Ž . erosion. Temporal changes in sediment yields are reconstructed by using caesium-137 Cs dating techniques. They indicate significant changes resulting from human activity and rainfall variability during the past four decades. Comparing reservoir sedimentation against erosion estimates on catchment soils, derived from measurements, indicates that 84% of eroded soil is delivered to the reservoir and the remainder is deposited in valley floor paddy fields. It is estimated that over 60% of sediment is contributed from arable land. The trap efficiency of small catchment reservoirs, such as Yiwanshui, may be an important control on sediment delivery to the main channel of the Upper Yangtze. q 2001 Elsevier Science B.V. All rights reserved.