The Haihe River, the largest water system in northern china, is formed by five large rivers -- the North Canal, the Yongding, the Daqing, the Ziya rivers and the South Canal, all of which converge near Tianjin to form the Haihe River, and then flow east and empty into the Bohai Sea at Dagukou. The 73-kilometer trunk of the Haihe channels more than 300 tributaries into the sea. The Haihe basin covers Beijing, Tianjin, the greater part of Hebei, and parts of Shandong, Henan, Shanxi and Inner Mongolia. It has a drainage area of 317,800 square kilometers, taking up 3.3% of the total area of China.     

The rainy season concentrates in a very short period in the Haihe valley. When heavy rains occur, the torrents of the numerous tributaries in the upper reaches rush into low-lying middle and lower reaches, where the river is narrow and there is only one outlet to the sea, causing it to burst onto the plain in floods. In 1963, the Haihe basin was hit by an unprecedented flood. In the following decade or more, comprehensive measures have been taken to bring the river under permanent control. More than 1900 reservoirs and 4,300 kilometers of dykes have been built, the river channels have been dredged, and 8 additional outlets (including Zhangwei New River, Ziya New River, Yongding New River and Chaobai New River) to the sea have been opened, raising the flood-diversion capacity to 25,000 cubic meters per second. Now the Haihe basin is basically free from the menace of floods.