Why does Dongting Lake change in size between summer and winter?

Dongting Lake in Hunan province experiences significant seasonal fluctuations in size because it functions as a massive retention reservoir for the Yangtze River.1

During the flood season, which lasts from June to October, the lake’s water level rises as it receives a substantial portion of the Yangtze’s waters, along with the drainage from almost the entire Hunan river system. This influx can increase the lake's water level by as much as 50 feet (15 metres) and expand its surface area by up to 7,700 square miles (20,000 square km).1

Conversely, from October to April, the lake discharges more water into the Yangtze than it receives. As the water level falls during this period, much of the lake's area recedes, leaving behind dry land. This natural cycle is essential for regulating flooding, though the lake's capacity has historically been affected by sediment deposition and land reclamation projects.1