Impact of cascade reservoirs construction in the upper Yangtze River on sediment inflow to Three Gorges reservoirs

Construction of reservoirs in the upper Yangtze River(Changjiang) has great impact on the incoming flow and sediment conditions of the Three Gorges reservoir(TGR).This paper analyzes the impact of 12000 reservoirs in the upper Yangtze on the sediment inflow to TGR,and predicts the trend of incoming sediment for the case of all the cascade reservoirs at their operation stage.Results show that about 180 millions ton(Mt) of sediment was trapped annually by these reservoirs in 1991-2005 and that about 60.1 Mt /yr and 50 Mt /yr or 33.8% and 27.7% of the total trapped was retained in the Jialing River basin and the Jinsha River basin,respectively.The sediment detention had reduced the TGR incoming sediment by 110 Mt /yr in 1991-2005.Relative to that before 1990,the reduction was increasing by 95.1 Mt /yr on average,or the total reduction in the TGR incoming sediment in this period was increased by 59.9%.With the planned large reservoirs at their operation stage in the near future,the sediment discharge to TGR would most likely be decreased to 90 Mt /yr,which is only 18.3% of the amount observed during 1956-1990.