Xuanwu Lake, also known as Houhu and Beihu, is located in Xuanwu District, Nanjing, Jiangsu Province. Bounded by Purple Mountain to the east, the Ming Dynasty City Wall to the west, Nanjing Railway Station to the north, and Fuzhou Mountain to the south, Xuanwu Lake stretches 2.4 kilometers from north to south and 2.0 kilometers from east to west. It has a surface area of 3.78 square kilometers and a reservoir capacity of approximately 5.5 million cubic meters.
Xuanwu Lake is a shallow lake, a remnant of the ancient Qinhuai Canal of the Yangtze River. It is the heart of Nanjing's urban water system and belongs to the Jinchuan River system. Its water is primarily supplied by rainwater from the northern foothills of Zhongshan (Purple Mountain). Connected to the Yangtze River via the Jinchuan, Pearl, and Qinhuai Rivers, Xuanwu Lake serves a variety of functions, including flood control and drainage, ecological landscape development, leisure activities for residents, tourism, and water replenishment for urban river ecosystems. Xuanwu Lake is one of the three famous lakes in the Jiangnan region. Historically, the area surrounding ancient Xuanwu Lake was the most habitable area in Nanjing, home to ancient cultural sites such as Beiyinyangying, Suojin Village, and Anhuai Village. Xuanwu Lake also served as an imperial garden lake and a training ground for naval exercises during the Six Dynasties period. It was also the site of the National Archives, where the Yellow Books were kept during the Ming Dynasty. Consequently, Xuanwu Lake is known as the largest inner-city park in the Jiangnan region, the largest imperial garden lake in China, and the last remaining imperial garden in Jiangnan. It is also known as the "Pearl of Jinling."
On August 19, 1928, Xuanwu Lake officially opened to the public as a park. In February 2005, Xuanwu Lake Park was designated a National AAAA-level Tourist Attraction by China's National Tourism Administration. In September 2008, Xuanwu Lake Park was designated a National Key Park by the Ministry of Housing and Urban-Rural Development of the People's Republic of China. In September 2016, Xuanwu Lake Park was designated a National Water Conservancy Scenic Area by the Ministry of Water Resources of the People's Republic of China.
The formation of Xuanwu Lake is primarily due to geological structure and the diversion of ancient river channels. The Xuanwu Lake lowland was formed by four major faulting events near Xuanwu Lake: the Zhongshan (Purple Mountain) thrust fault, the Tianbaocheng fault, the Fuguishan fault, and the Taipingmen fault. While the first three faults were indirectly linked to the formation of the Xuanwu Lake lowland, the fourth was directly linked. The Taipingmen fault, which plunged its western flank downward, created the Xuanwu Lake lowland and allowed water to accumulate and form the lake. Therefore, scholars Bi Qingchang and Wang Jianxiang believe that Xuanwu Lake is a natural lake, formed in the middle Tertiary. However, Professor Li Liwen of Nanjing Normal University believes that the origin of Xuanwu Lake requires a comprehensive analysis of geological structure, lithology, and human influence. For example, the Xuanwu Lake area lies at the tectonic boundary between the Ningzhen and Ningwu regions, at the ventral end of the Zhongshan-Mufushan complex syncline. This zone is characterized by a crisscrossing fault system, forming a fractured zone. Within this fractured zone, igneous intrusions developed through weathering and erosion, forming a river and forming the original lake basin. Due to the accumulation of silt, after millions of years of erosion, it gradually dissolved into a low-lying area with accumulated water, forming a lake. The lake then spread outwards, eroding the surrounding area with waves, and the area continued to expand, eventually giving birth to Xuanwu Lake.