Sediment delivery to the Three Gorges 2: Local response

Information on soil erosion and sediment yields is very limited in the imme diate vicinity of the Three Gorges Project (TGP), although the inceptisols developed on Triassic shales and sandstones in this area are known to be hi ghly susceptible to erosion. Estimated sediment yields for a small reservoi r catchment (Yiwanshui) in Changshou County, downstream of Chongqing, are h igh (around 3500 t km(-2) a(-1)) 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 so il 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 estimate d, using small reservoir data and sediment load measured at hydrological st ations. These are comparable with previous estimates but there is much temp oral variability in sediment supply from different parts of the Three Gorge s region, suggesting the importance of local impact on sediment flux. Reset tlement of agricultural activity around the Three Gorges Reservoir is likel y to further enhance soil erosion. Temporal changes in sediment yields are reconstructed by using caesium-137 (Cs-137) dating techniques. They indicat e significant changes resulting from human activity and rainfall variabilit y during the past four decades. Comparing reservoir sedimentation against e rosion estimates on catchment soils, derived from measurements, indicates t hat 84% of eroded soil is delivered to the reservoir and the remainder is d eposited in valley floor paddy fields. It is estimated that over 60% of sed iment is contributed from arable land. The trap efficiency of small catchme nt reservoirs, such as Yiwanshui, may be an important control on sediment d elivery to the main channel of the Upper Yangtze. (C) 2001 Elsevier Science B.V. All rights reserved.