[1] Water and sediment supplies from ungauged areas were calculated and combined with datasets from gauging stations to establish sediment budgets. Using sediment budgets and regression relationships, the influence of the Three Gorges Dam (TGD) on downstream delivery of sediment was quantified. We found that 151 mt/yr (1 mt = 10 6 tons) of sediment has been retained by TGD since it began operation (2003 -2005). In response to this, significant erosion has occurred in the downstream riverbed. This erosion did not offset the sediment lost in the reservoir, and the sediment flux into the estuary decreased by 85 mt/yr (31%). This decrease has lead to conversion from progradation to recession in the delta front. In combination with other anthropogenic impacts, TGD was expected to decrease the sediment flux into the estuary for centuries, which is of great importance for delta ecosystem and human development.

No abstract

[1] To examine the influence of deposition and erosion within the main river channel and large lakes on sediment delivery to the estuary at the mouth of the Yangtze River, we established sediment budgets from time series data for 1956-2004 of annual sediment load gauged at 24 stations and estimated data for ungauged areas. Ungauged areas supplied approximately 17% of the total sediment to the estuary, while an average of 34% of sediment is stored in the main river channel complex. Prior to the operation of the Three Gorges Dam (TGD) in 2003, 88% of deposition occurred within a 700-km section immediately downstream from the outlet of the Three Gorges area, including the main channel and a linked lake. Since the operation of the dam, however, deposition has migrated into the Three Gorges area, and downstream erosion has been observed. Patterns of deposition and erosion depend mainly on sediment input, except for the lower 500 km of the river where water discharge is the governing factor. Although sediment supply from the subbasins to the main river system will presumably experience a further decrease because of construction of new dams and afforestation, sediment flux to the estuary is unlikely to fall below 100 Â 10 9 kg/yr over the next 50 years because sediment availability downstream of the TGD will not be a limiting factor over this timescale.

No abstract