The Hai River as it exists today is, in a sense, an artificial creation. Before the Grand Canal was constructed, the Wei, Ziya, Yongding, and Bai rivers flowed independently into the sea along separate courses. The Grand Canal cut through the lower reaches of all of them, fusing their outlets into a single estuary and creating the modern Hai River. The Northern Canal, which forms the northern terminus of the Beijing-Hangzhou Grand Canal system, connects with the Chaobai River at Tongzhou. The Southern Canal draws from the Wei River at Linqing. Through these connections, the Hai River links to both the Yellow and Yangtze river systems, making it a node in the largest engineered waterway network in human history. The river's very name is relatively recent: it first appeared at the end of the Ming Dynasty. During the Song Dynasty it was called the Border River; under the Jin and Yuan, it was the Straight Gu River and then the Great Gu River.

The Hai River's strategic position made it a focal point for conflict whenever foreign powers sought access to Beijing. On May 20, 1858, Anglo-French forces attacked the Taku Forts at the river's mouth during the Second Opium War, capturing them and forcing the Treaties of Tianjin that opened the city to foreign trade. In 1863, seagoing ships could still navigate upriver to Tongzhou, though the river's crooked course made it difficult for large vessels. Then came the Boxer Rebellion. On June 15, 1900, Imperial Chinese forces deployed electric mines in the river to prevent Eight-Nation Alliance warships from advancing upstream. The allied navies attacked anyway, capturing the forts again in a bloody overnight assault that helped trigger the siege of the foreign legations in Beijing.

Like the Yellow River, the Hai is exceedingly muddy. The powdery soil of the North China Plain dissolves into the current and deposits in the lower reaches, sometimes raising the riverbed above the surrounding landscape and creating conditions for catastrophic flooding. The problem is compounded by geography: five major tributaries converge into a single shallow outlet. When heavy rains come, there is only one way out, and it is often not enough. Because both Beijing, China's capital and second-largest city, and Tianjin, its third-largest, sit within the Hai River basin, the consequences of flooding have always been severe. Reservoirs have been built and artificial channels dug to divert excess water directly to the sea. The Chaobai River has been permanently rerouted into the Chaobai Xin River, severing its ancient connection with the Northern Canal.

The modern threat to the Hai River is not excess water but its absence. Industrial and urban development across the basin has dramatically reduced water flow. Many smaller tributaries are dry for most of the year, and even some major tributaries have dwindled to intermittent streams. As the volume of water drops, pollution concentrates. The river that once carried imperial grain shipments and gunboat flotillas now struggles to sustain the basic ecological functions of a living waterway. The South-North Water Transfer Project, one of the most ambitious hydraulic engineering schemes in history, aims to divert water from the Yangtze basin to alleviate the crisis. Whether it will be enough to restore the Hai to something resembling its former self remains an open question.