There is a great difference in duration of floods of each river in northeast of China. A short duration of flood lasts only two days, and the long one can reach 60 days. The main factors that affect the duration of flood are rainstorm duration, rainfall distribution, catchment area, soil vegetation condition, topography and river morphology.
The floods of Yalu River basin, southeast coastal rivers, Liaohe River, Tumen River, second Songhua River, Lalin River, tributaries of Songhua River, Nenjiang River have lasted 3~7 days. The floods in most tributaries of Nenjiang River lasted 10~15 days, but the flood blow Ayanqian station lasted 30~60 days. The flood in Songhua River lasted 40~60 days. The flood in Suifen River and Muling River lasted 15~22 days. The flood lasted 30~50 days in the Heilongjiang River and the Wusulijiang River.
The majority of medium and small rivers flood is unimodal. In the long rivers, due to the different stages of the rainfall and the different timing of tributaries water, the flood often is bimodal.
The flood hydrograph was asymmetrical, of which the rising period lasted shorter than the falling period, and the ratio of them is related to the soil, and local vegetation conditions. The ratio of rising and falling period changes in the range of 2.2~5.0.
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