Wetlands function as natural sponges, that traps and slowly releases surface water, rain, snowmelt, groundwater and flood waters. Trees, root mats, and other wetland vegetation are also slowing the speed of flood waters and distribute them more slowly over the floodplains. This combined water storage and braking actions, lowers the flood heights and reduces erosion.
Wetlands within, and downstream of urban areas are particularly valuable. They are  counteracting the greatly increased rate and volume of surface and water runoff from pavement and buildings. The holding capacity of wetlands, helps control floods and prevents water logging of crops. 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. The bottomland hardwood- and riparian wetlands along the Mississippi River, once stored at least 60 days of floodwater. Now they store only 12 days because most have been filled or drained.