The Yangshui River, a tributary of the Wu, runs under three limestone spans in Xiannüshan Town, and the Chinese names of all three refer to dragons: Tianlong, the sky dragon, Qinglong, the azure, and Heilong, the black. What makes the group unusual is the spacing. From the upper end of Tianlong Bridge to the lower end of Heilong Bridge the straight-line distance is only 1,500 metres, so three bridges of this size follow one another on a single river. Chinese descriptions of the site say that this succession occurs nowhere else in the world. It also explains a limitation: measured span by span, they are not the longest natural bridges anywhere.
From the journal
Three limestone spans on one river, and the hollows between them
How the Yangshui River left three bridges standing in succession at Xiannüshan, and why the two collapse hollows between them matter as much as the spans.
In the same city · Wulong ↗Three bridges on one tributary
The two hollows between the spans
The bridges are not evenly spaced. Between Tianlong and Qinglong lies the Qinglong tiankeng, and between Qinglong and Heilong the Shenying tiankeng. Both are collapse hollows of the kind the Chinese call tiankeng, and both are large: 276 to 285 metres deep and 300 to 522 metres across. Read as one profile, the river runs bridge, hollow, bridge, hollow, bridge, which is why the conservation area is described as a three-dimensional karst landscape of natural bridges, hollows, rifts and caves rather than as three separate monuments. The floor of each hollow is the river bed, and the walls are the limestone the water has been removing.
What the World Heritage listing covers
The Three Natural Bridges are the nucleus of a conservation area of about 22 square kilometres. Inside it are the Yangshui River karst canyon, the Longshui Gorge rift, two more tiankengs at Shiyuan, several named caves and two hidden streams that go underground. The site is rated a national 5A tourist area, and in June 2007 it entered the UNESCO World Heritage list as part of the South China Karst-Wulong Karst. Wulong's karst is counted among Chongqing's three World Natural Heritage sites. The wider park is a national geology park, and the Qingkou cluster at Houping, a separate part of the same listing, is the only tiankeng group known anywhere that is attributed to surface water erosion.