Floods and flash floods 


Flooded forest, Cambodia © Flickr/Dominiqueb

Floods occur when the rate of water supply exceeds the capacity of stream-channel drainage such as during periods of heavy rains and rapidly melting snow and ice. They can be triggered by cyclones, severe thunderstorms, tornadoes and monsoons or can result from the building of dams (i.e. by beavers) or by dam breaks caused by general failure, ice jams, landslides, or by tectonic and other geological processes. Excessive rainfall on saturated soils in flat areas can also create floods. In coastal areas, storm surges caused by cyclones and tsunamis, or by a combination of high river flows and back-water effects as a result of high tides, can also cause flooding.

The ecological damage of regular flooding may be minor as floodplain plants and animals are well adapted to such conditions. In addition, the mechanical force of floodwater is not typically adequate to increase plant mortality rates, especially when flooding occurs in spring before bud break of deciduous trees. Oxbow formation, ice scouring, and bank erosion may cause the death of some trees and changes in the landscape mosaic, but impacts are generally affected only over limited areas. Floods caused by waterlogging of large, flat areas, however, can persist for several days and cause damage to trees and forests.

Flash floods can occur after heavy storms or after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Such events may have much more impact on forests, especially in areas not accustomed to high waters. 

last updated:  Friday, June 28, 2013