The Huai River-Qinling Mountains line is regarded as the geographical dividing line between North China and South China. This line approximates the 0 degree January isotherm and the 800 mm isohyet in China.
The Huai river has a length of 1,078 kilometers and a drainage area of 174,000 square kilometers.
Historically, the Huai River entered the Yellow Sea at Yuntiguan ("Escalade Pass") through a broad and level lower course. It was long used to irrigate the surrounding farmlands, and was the centre of an extensive network of canals and tributaries. Beginning in 1194, however, the Yellow River to the north repeatedly changed its course southwards to run into the Huai River. The resulting silting was so heavy that after the Yellow River changed back to its northerly course for the most recent time in 1897, the geography of the Huai River basin was changed significantly by the creation of new high lands, lakes, and the built up silt of the Yellow River's historical southern course. As a result, water from the mid-section of the river could not easily flow into the lower section, while water in the lower section could not find an outlet to the sea. The problem was exacerbated in the Second World War, when the government, in an attempt to check the pace of the Japanese invasion, flooded the lower Huai basin by opening the Yellow River's southern levee. The mainstem of the Yellow River flowed through the levee breach for the next nine years, further disrupting the Huai river system.
The result of these changes was that water from the Huai River pools up into into Lake Hongze, and then runs southwards towards the Yangtze River. Major and minor floods occurred frequently, with the area suffering droughts in between floods. In the 450 years to 1950, the Huai River saw, on average, 94 major floods per century.
Attempts to solve the Huai River's problems have focussed on building outlets for the Huai River into the Yangtze River and the sea. Currently, the major part of the river's waterflow enters the Yangtze River via Lake Hongze. The North Jiangsu Irrigation Main Channel also diverts some of its water along its old historical course to the sea, and is planned to be upgraded with a new parallel channel. Several former distributaries also carry some water to the sea.