How Wetlands Reduce Flooding
A wetland is an area where water covers the soil for at least part of the year. Some cities once drained wetlands to create space for roads and buildings. Recently, however, planners have begun restoring them. A restored wetland can hold rainwater that would otherwise flow quickly into streets and drainage pipes.
Wetland plants slow moving water, while the uneven ground creates small pools. During a heavy storm, these features spread water across a wider area. As a result, the amount entering a nearby stream at any one moment can be lower. The wetland does not eliminate a flood, but it can reduce the peak flow, the highest rate at which water passes a point.
Wetlands can also improve water quality. Soil and plants trap some sediment before it reaches the stream. Researchers often compare water samples upstream and downstream to see whether this effect occurs. They do not assume that every restored site works equally well: the size of the wetland, the local soil, and the path water takes through it all matter.
Restoration has limits. Land used for a wetland cannot also be used for housing, and a small site may provide little protection during an extreme storm. For this reason, city planners usually combine wetlands with drains, flood barriers, and rules that limit construction in the most vulnerable places.