Yunhu formerly known as Hengshan Reservoir is a large national reservoir, with a water catchment area of 154 square kilometers, surrounded by mountains, clear water quality and beautiful natural scenery. It is a rare tourist resort. Hengshan Reservoir is located in the southwest mountainous area of Yixing, Jiangsu Province. It was built in 1958 and built in 1969. It is a water conservancy hub for flood prevention, irrigation and water supply, combining aquaculture, power generation and comprehensive utilization of the tertiary industry. The reservoir is designated as the whole ecological water source protection area of Yixing City. The total reservoir capacity of the reservoir is1.12 billion cubic meters, the basin area is 154.8 square kilometers, the dam is 4090 meters long, one three-hole spill gate, one hole wide 4.6 meters, and the dome height is 32 meters. The design discharge is 485 cubic meters/second. There is a water transmission culvert on both sides of the dam. The daily water supply is 250,000 cubic meters, and the water quality is between the Ⅰ and Ⅱ categories for a long time. The reservoir was retrenched in 2001 and completed in September 2003, reaching the standard of 2000.