Sediment delivery to the three gorges:: 1. Catchment controls

Abstract

The paper examines sediment yield and its response to catchment disturbance and environmental variables in the Upper Yangtze basin, where the attention of environmentalists has been drawn to the Three Gorges Project (TGP). Information about the source and conveyance of sediment from the catchment area to the Three Gorges Reservoir has implications for management strategies. Methodologies for establishing the relationships between land cover, climatic and topographic variables with sediment yield are introduced. The analysis uses a sediment load data set, containing 250 stations with up to 30 years of measurement, a 1×1 km resolution land cover database and variables extracted from various geodatabases. The mean sediment load delivered from the Yangtze upstream of Chongqing is 318 Mt a −1, but the contribution from the Jialing tributary is higher in terms of specific sediment yield at 928 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 basin. 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 illustrates that significant increases in sediment yield have occurred over about 8% of the catchment area while about 3% have experienced decreasing sediment yields. The latter are associated with major reservoir schemes on the tributaries of the Yangtze. When the spatial pattern of sediment yields within the basin is analysed, “natural” climatic and topographic factors explain 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 not provide additional explanation of spatial variability. Examination of the response of sediment delivery to catchment disturbance and environmental variables provides an illustration which may have some lessons for the management of the sedimentation problem in the Three Gorges Reservoir and a basis for modelling future changes in sediment delivery.