From Envirowiki

A Flood is a swelling of a water body (usually a river or stream, but sometimes also lakes and coastal waters) so that they rise, and sometimes break their banks, and flow out onto the flood plain. Floods occur naturally after long or intense periods of rain. Floods carry nutrient-filled sediment and silt to floodplains, which increases the soil productivity. Floods also help to refresh stagnant parts of rivers.

Large dams destroy the natural flood cycle, stopping the flow of nutrients down stream.

Flooding can be devastating for human settlements and their domesticated animals and agriculture, if care is not taken to have emergency flood procedures. Even with those procedures, flash flooding can occur, when heavy rain falls upstream, and river waters can rise extremely quickly.

Climate change is likely to cause more frequent extreme flooding events. This is already occuring somewhat in europe.