Effect of the evolution of river-lake relation on the water environmental capacity of the outlet reach of Xiangjiang River

Based on the analysis of the evolution of river-lake relation of the middle and lower reaches of Yangtze River, the water level variation of Dongting Lake was analyzed. With one-dimensional hydrodynamic and water quality model, the response mechanism of hydrodynamic characteristics of the outlet reach of Xiangjiang River to the water level variation of Dongting Lake was further analyzed and the effect of flow characteristics variation on the water environmental capacity was evaluated quantitatively. The result shows that under the new pattern of river-lake relation, especially after the construction and operation of the Three Gorges Project, water level of the Dongting Lake is showing a declining trend, which has led to a decrease in the water velocity and water level at the estuary of Xiangjiang river (Haohekou) with respectively 0.03 m/s and 2.5 m. The water environmental capacity of the outlet reach of Xiangjiang River was also decreasing, among them CODMN decreasing by 5566.5 tons/yr and NH3-N 2999.6 tons/yr.