Yongding River

The Yongding River (zh-stp|s=永定河|t=same|p=yǒngdìng hé)(Wade Giles: Yungting River) is a river in northern China. It is one of the main tributaries in the Hai River system and is best known as the largest river to flow through Beijing Municipality. The river is 650 km in length and drains an area of 47,016 sq. km. It emerges from the Guancen Mountains of Ningwu County in Shanxi Province, where it is known as the Cangqian River (桑乾河) and flows northeast into Inner Mongolia and then heads southeast into Hebei Province. In Huailai County, it fills the Guanting Resevoir, the biggest resevoir serving Beijing and takes on the name Yongding. It enters Beijing Municipality through the mountains of Yanqing County, northwest of the city centre and descends into the flatlands in Mentougou District. The river eventually flows back into Hebei Province and then into Tianjin Municipality, where it meets the Hai River just before the latters empties into the Bohai Sea at Tanggu.

Historically, the river was notorious for its flash floods and course changes. The river has taken at least three major courses through Beijing. According to the earliest historical records, the river originally flowed northeast from Babaoshan toward what is now the Purple Bamboo Park in Haidian District and into Wenyu River. In the Western Han Dynasty, the river flowed south from Deshengmen through the present-day downtown Beijing, including Qianmen and Honqiao and Longtan Lake, before leaving to the southeast. At that time, the city was situated to the southwest of today's city centre. In the Liao Dynasty, the river moved southwest to its present course, and the Lugou Bridge was built over it in 1189. When Marco Polo visited the city during the Yuan Dynasty, he crossed the Yongding River on the Lugou Bridge, which became known as the Marco Polo Bridge.

The river known colloquially as the "Wuding River" or the "River of Instability." In 1698, the Qing Dynasty government reinforced the river banks and rendered the current course permanent. The river thus became known as the "Yongding River" or the "River of Eternal Stability."