Sediment delivery to the three gorges: 1. Catchment controls

The paper examines sediment yield and its response to catchment disturbance and environmental variables in the Upper Yangtze basin, where the attentio n of environmentalists has been drawn to the Three Gorges Project (TGP). In formation about the source and conveyance of sediment from the catchment ar ea to the Three Gorges Reservoir has implications for management strategies . Methodologies for establishing the relationships between land cover, clim atic and topographic variables with sediment yield are introduced. The anal ysis uses a sediment load data set, containing 250 stations with up to 30 y ears of measurement, a I X I km resolution land cover database and variable s extracted from various geodatabases. The mean sediment load delivered fro m the Yangtze upstream of Chongqing is 318 Mt a(-1), but the contribution f rom the Baling tributary is higher in terms of specific sediment yield at 9 28 t km(-2) a(-1). Long-term sediment yield at Yichang has not exhibited an upward trend despite the evidence for increased soil erosion within the ba sin. Examination of sediment response to catchment disturbance and spatial variability in relation to controlling variables has been undertaken in an attempt to predict future sedimentation impacts. Time series analysis illus trates that significant increases in sediment yield have occurred over abou t 8% of the catchment area while about 3% have experienced decreasing sedim ent yields. The latter are associated with major reservoir schemes on the t ributaries of the Yangtze. When the spatial pattern of sediment yields with in the basin is analysed, "natural" climatic and topographic factors explai n most of the variability in the relatively sparsely populated western part of the Upper Yangtze basin, but do not afford very good prediction in the more populated eastern part. Incorporation of land cover information does n ot provide additional explanation of spatial variability. Examination of th e response of sediment delivery to catchment disturbance and environmental variables provides an illustration which may have some lessons for the mana gement of the sedimentation problem in the Three Gorges Reservoir and a bas is for modelling future changes in sediment delivery. (C) 2001 Elsevier Sci ence BN. All rights reserved.