The hydrological changes in the Lower Jingjiang reach is related to the complex exchange relationship between Dongting Lake and Yangtze River. After the construction of the Three Gorges Dam (TGD), the change of the base level of lake Exit makes the lake-to-river supporting and falling effect adjustment, which could have an adverse impact on flood control & draught resistance and ecological environmental protection. Based on the variation of water surface slope, the supporting and falling ratio is proposed to characterize lake-to-river effect. The data at Jianli, Luoshan and Qilishan station from 1991 to 2017 are used to analyze the supporting and falling effect and its variation characteristics. The rationality of gradient change before and after impoundment is explained theoretically by using balanced gradient formula. The results showed that:After impoundment, the water surface slope of Jianli-Chenglingji and Chenglingji-Luoshan increased. The minimum monthly slope of Jianli-Chenglingji reach was delayed from 1991-2002 to 2003-2007 to 2008-2017. The corresponding water level in Chenglingji increased, while the confluence ratio decreased, and the supporting effect of lake-to-river was weakened. The lake supported the river comprehensively during 1991-2017 periods. The reason for supporting and falling ratio reduction is the weakening of supporting effect rather than the enhancement of falling effect. After the flood season (September-October) during three periods, the lake showed a falling effect on the river, while from 2008 to 2017, the falling effect of the lake also occurred in the dry season (January-March). Supporting and falling ratio is affected by the confluence ratio during three periods. When the confluence ratio is less than 0.5, the lake is liable to the falling effect on the river. When the confluence ratio is greater than 1, the supporting effect increases. When the discharge in the Yangtze River is less than 6 000 m3/s, the Dongting Lake acted as a supporting force to the Yangtze River. During 1991-2002, the supporting and falling effects were mainly affected by the Dongting Lake outflow, while during 2008-2017, they were mainly affected by the discharge in the Yangtze River. The research results can provide scientific basis for river regulation, water ecology and water resources utilization in the lower Jingjiang River.
