Variation of runoff and sediment inflows to the Three Gorges Reservoir: Impact of upstream cascade reservoirs

The construction of cascade reservoirs upstream of the Three Gorges Reservoir (TGR) has been greatly intensified in the 21st century. While research attention is dedicated to the downstream effects of the TGR, few studies have been focused on the radical change of upstream inflows to the TGR. In this study, we reveal the impact of upstream cascade reservoirs on the annual runoff and sediment load inflows to the TGR based on the data in 1956-2016. The measured sediment load at control hydrological stations shows a drastic decrease (59%) while the runoff only had a slight decrease (6%). The similarity of these characteristics for that measured at Yichang indicates the leading effects of TGR inflows on its outflow. We develop a macroscopic quantification method utilizing double mass curves to separate the contribution from climate effects and human activities. Results show that precipitation (climate factors) dominated the runoff change, while human activities were the main driving factors for sediment load reduction in the studied area. In the four sub-basins upstream of the TGR, the sediment retention induced by large reservoirs was the major reason for sediment load reduction, especially in the Jinsha River Basin and the Jialing River Basin, which contributed 70% (or 347 Mt/yr) of the total sediment retention in the whole upper Yangtze River Basin (YRB) since 2015, much larger than that of the TGR (19% or 94 Mt/yr). As the total storage capacity of the cascade reservoirs increased to more than twice the TGR's capacity in the 2010s, the regression relationship between runoff and sediment load broke down. The unprecedented situation in the upper YRB may have profound impacts on the morphology and ecology of the TGR as well as the downstream Yangtze River including its delta.