At the beginning of Emperor Qianlong’s reign, the number of traditional Chinese gardens in Haidian District gradually grew, and the water yield used by the gardens increased rapidly day by day. The water used by these gardens was some sourced from Wanquan River and much sourced from the West Lake where the water came from Yuquan Mountain.

In 1750, Emperor Qianlong ordered his officials to broaden the West Lake to prevent the water from Xishan Mountain, Yuquan Mountain and Shouan Mountain, to prepare for the 60th birthday of his mother. In fact, this was an excuse. He also ordered to dig two other lakes so as to guarantee the water use of the imperial gardens and provide water for the surrounding fields. In the Han Dynasty, Emperor Wudi once dug a pool named Kunming Pool to practice water armies. Therefore, this emperor changed the lake’s name to Kunming Lake, and also changed the hill Wongshan Hill to the Longevity Hill.

Source: baike.baidu.com