The researchers studied water quality in Dongting Lake after the Three Gorges Dam was built. They found that nitrogen levels increased before the dam, but decreased after. Phosphorus levels decreased after the dam. The main source of pollution was nutrients from tributaries. Water flow and levels also affected water quality. Tributaries contributed more to pollution than channels. After the dam, channels contributed less due to reduced nutrient and sediment flow. To improve water quality, controlling pollution from tributaries and monitoring lake conditions are crucial.