river in east-central china that drains the plain between the Huang He (Yellow River) and the Yangtze River (Chang Jiang). The river has a length of 660 miles (1,100 km) and drains an area of 67,000 square miles (174,000 square km). It is fed by numerous tributary streams rising in the Funiu, the Tongbai, and the Dabie (Dabie Mountains) mountains, which, with their extensions into Anhui province north of the Yangtze, form its southern watershed. The Huai River flows eastward to discharge into Lake Hongze (Hongze Lake) in Jiangsu (Kiangsi) province. In ancient times this lake was much smaller than at present, and the Huai River flowed from it into the sea roughly on the line of the modern Guan River, south of Lianyungang. The river's mouth was, however, blocked by silt, and so the water from Lake Hongze drained away through the string of lakes in eastern Jiangsu into the Yangtze near Yangzhou.

In the north a series of tributaries flows northwest to southeast from a low watershed almost on the southern dikes of the Huang He. From time to time the Huang He has flowed through the north of the Huai drainage basin and has discharged into the Huai or even, on occasion, first into the Huai and then into the Yangtze. The drainage of this flat and featureless plain has been a perennial problem, particularly since the 1850s, when the Huang He, which had previously discharged into the sea at Haizhou Bay, again coursed north of the Shandong Peninsula. As a result, much of the drainage into its lower course was diverted into the Huai River, leading to continual flooding.

In the 1930s part of the Huai River system was dredged, and an artificial channel protected by flood barrages was cut from Lake Hongze to the sea. In 1938, during the Sino-Japanese War, the Chinese army, in an attempt to block the Japanese southward advance through the North China Plain, blew up the dikes of the Huang He near Zhengzhou, flooding a vast area in Henan province.

Extensive work to control the Huai took place after World War II. The dikes were repaired, and the Huang He returned to its old course through northern Shandong (Shantung) province. In 1951 work began on a comprehensive water-conservancy project for the Huai basin. The Subei Canal, the outlet canal from Lake Hongze to the sea, was reconstructed, and an alternative outlet to the sea was also completed. At the same time, the repair and improvement of the Grand Canal also improved the drainage of Lake Hongze to the south. In the early 1950s the headwaters of the Huai and its western tributaries in the Funiu and Tongbai mountain ranges were controlled by constructing many large retention dams. In 1957 a second stage of flood control began on the southern tributaries. After 1958 the area south of the Huai was incorporated into a large coordinated irrigation system.

In the late 1960s government attention shifted to work on the New Bian Canal north of the Huai, although development in the south still continued. By the early 1970s the Huai's northern tributaries had been joined to the New Bian Canal, which provided more effective flood control in the northern Huai plain. From the early 1980s the Huai was navigable by small ships above Huainan, while the Subei Canal provided a navigable outlet to the sea. River traffic from the Huai could also join the Grand Canal, providing a water transport route north to the Huang He and south to the Yangtze River.