Influence of the impounding process of the Three Gorges Reservoir up to water level 172.5 m on water eutrophication in the Xiangxi Bay

Abstract

The water level of the Three Gorges Reservoir (TGR) rose to 172.5 m in two stages from September to November, 2008. The hydrodynamic parameters and water quality parameters (e.g. flow velocity, temperature, turbidity and nutrient content) had been continuously monitored to reveal the influence of the impounding process of the TGR on the water eutrophication in the Xiangxi Bay (XXB). The maximal daily rise of the water level of the TGR was about 2.38 m. The water exchange between the mainstream (the Changjiang River) and the XXB of the TGR was enhanced by density current, and its main characteristics were a density-stratified flow in different directions. The water exchange reduced the differences of the water quality parameters (for example, temperature, turbidity, nutrient level, pH, etc.) between the mainstream and the XXB during the impounding process. Meanwhile, the water stability index in the mainstream remained small but it was lowered in the XXB. The main causes included the dilution by water of lower concentration of the chlorophyll a (Chl. a) entering from the mainstream, the decline of the water stability, and the increase of suspended silt, which induced the Chl.a to decrease in the XXB during the impounding process. The grade of the water eutrophication changed from middle eutrophic level to mesotrophic state in the process. However, as available nutrients were imported into the XXB from the mainstream during the impounding process the risk of water eutrophication increases in the XXB in the future.