Floods can occur anywhere, but urban areas experience increased frequency and severity of flooding events. This is a result of the high percentage of impervious surfaces, such as pavement and roofs, which force water to "run" rather than soak into the soil. In response to the increased "run-off" of water, most developed areas have extensive systems of gutters, drains and pipes to quickly move water "away" to nearby streams. This results in the rapid rise of water levels and increased velocity, which easily result in localized or even regional flooding.
Trees in urban areas can reduce these sudden waves of water by slowing rainfall through a leafy canopy, giving time for more water to infiltrate soils. This mitigates heavy rainfall by essentially spreading out the rain event, resulting in less and slower runoff. With increased tree cover, the risk of flooding is reduced. When flooding does occur, the volume and speed of flooding waters will also be reduced.