Erhai Lake, historically known as Kunming Pool, Er River, or Yuyu Ze, is a lake located on the upper reaches of the Xi'er River (a tributary of the Yangbi River, which in turn flows into the Lancang River). Situated in Dali City, within the Dali Bai Autonomous Prefecture of Yunnan Province, it is the second-largest lake in Yunnan, surpassed only by Dianchi Lake. The lake generally covers an area of about 246 square kilometers (some sources say 252 sq km) with a storage capacity of approximately 2.95 billion cubic meters. It is elongated in shape, stretching 40 kilometers from the southern end of Eryuan County in the north to Xiaguan in Dali City in the south. Formed at the end of the Ice Age, Erhai is a plateau tectonic fault lake created primarily by subsidence erosion, sitting at an altitude of 1,972 meters.
Part of the Lancang River basin, Erhai serves as the source of the Xi'er River. The lake water flows through Xiaguan in Dali's urban area via the Xi'er River, eventually merging westward into the Yangbi River.
Erhai Lake serves seven major functions: water supply, agricultural irrigation, power generation, climate regulation, fisheries, shipping, and tourism. The scenic environment is stunning, with the Cangshan Mountains lining the west like a screen and the Yu'an Mountains encircling the east.
In 1981, the Cangshan-Erhai Nature Reserve was established with the approval of the Yunnan Provincial People's Government. It was upgraded to a national-level reserve in 1994. The reserve covers 79,700 hectares and focuses on protecting the plateau freshwater lake ecosystem, aquatic flora and fauna, the natural landscape of the transition zone between northern and southern species, and glacial relics.