Study on the Water Level–Discharge Relationship Changes in Dongting Lake Outlet Section over 70 Years and the Impact of Yangtze River Backwater Effect

The hydrological characteristics of the river–lake connecting section are determined by their interaction and studying them can help to understand the changing relationship between these two water bodies over time. The Lujiao–Luosan section is the connecting section of Dongting Lake and the Yangtze River, and the hydrological data for this section over the past 70 years has been analyzed. It has been found that the lowest water level is consistently rising at the same discharge at Chenglingji station, which is the joint point of Dongting Lake and the Yangtze River. While this could alleviate the drought situation in the Dongting Lake area during dry seasons, it could pose a more significant flood-control challenge during high water levels in the flood season. The water surface slope shows a decreasing trend especially during the dry season, except for the high flood period (July–September), which indicates that the water slope in the connecting section of Dongting Lake has become flatter. The backwater effect of the Yangtze River on Dongting Lake becomes increasingly stronger as the water surface slope difference between the Chenglingji–Luoshan section and the Lujiao–Chenglingji section changes from negative to positive between January and April.