Is in the flood season.
The change of river runoff mainly depends on the recharge conditions of the river. The recharge sources of rivers in China are rain, melting ice and snow, groundwater and so on. Rain is the main supply. Some rivers can form spring floods, but they generally do not cause disasters.
Due to the difference of rainfall time, the flood season is not consistent. Rivers south of the Yangtze River will flood in early summer.
In the central and eastern regions, July and August when rainfall is concentrated are easy to cause floods.
"Ice flood" is a phenomenon in the special geographical environment when the rivers in the north thaw in spring. In some northern rivers, some reaches flow from south to north, such as Ningxia in Gansu, Inner Mongolia and Heilongjiang in the Yellow River Hetao. When it thaws in spring, the upstream thaws first, and the ice floes flow down the river, but the downstream has not thawed, which leads to the ice floes blocking and the water level rising. Moreover, ice floes are more likely to cut banks and cause floods. If this happens, blasting is usually used to eliminate the danger.
Rivers in northern China are mainly threatened by ice flood, with the Yellow River in March and Heilongjiang later.
The flood season is from May 1 day to1October 20th1day every year. The rainfall in Shanghai is obviously more than that in other months, so the water level of the river is higher than that in winter. But the river level in Shanghai is greatly affected by the tide. In May-June, due to the large size of tidal force and high tide level, it is customary to put May-June 10.