The lakes in the middle part of the Yangtze River help supplement water during extreme drought years. In 1978, before the Three Gorges Dam was built, Dongting and Poyang Lakes provided 30% and 14% of the water at Datong station. After the dam was built, discharges increased, with the lakes supplying 28% and 22% of the water in 2006. The lakes play a key role in maintaining water levels during drought years along the mid-lower reaches of the Yangtze River.