Flood Control Benefits of Wetlands

Many factors contribute to flood conditions and affect the severity of damage including: precipitation, slope, land use, soil type, water table level, amount of impervious surface, climate, and dams.  Wetlands are considered ìnatural spongesî and have tremendous capacity to act as natural flood control.  When rivers overflow during rain events, wetlands help absorb and slow floodwaters.  This ability to control floods can significantly prevent property damage and loss and can save lives.  Each wetland has itís own hydrology and holds water differently.  Some wetlands are ìflow-throughî wetlands.  However, when water flows through a wetland, it is slower than when it rushes down a stream.  Studies have found that when wetlands are destroyed, flooding increases significantly.

In contrast to narrower streambeds and channels, the wider open areas in wetlands and their vegetation can slow the speed of floodwaters.  The floodwaters then continue more slowly into a river or stream, evaporate into the atmosphere, and recharge the groundwater. Wetlands also function to trap sediment and debris as the speed of the water slows and the debris settles out.   By reducing the rate and amount of storm water entering the river or stream, wetlands lessen the destructiveness of the flood.

Wetlands within and downstream of urban areas are particularly valuable, counteracting the greatly increased rate and volume of surface water runoff from pavement, buildings and other impervious surfaces.  The holding capacity of wetlands helps control floods and can act as a natural storm water retention pond.  Preserving and restoring wetlands, together with other water retention, can often provide the level of flood control otherwise provided by expensive dredge operations and levees. 

References Used:

Baylands Ecosystem Habitat Goals, SF Bay Area Wetlands Ecosystem Goals Project
Feasibility Analysis: South Bay Salt Pond Restoration, Siegel and Bachand
Turning Salt into Environmental Gold, Save the Bay
Wetland Restoration, EPA fact sheet 843-F-01-002e
Wetland Overview, EPA fact sheet 843-F-01-002a
Restoring the Estuary: An Implementation Strategy for the San Francisco Bay Joint Venture.  January 2001