Stormwater runoff is precipitation (rain or snowmelt) that flows across the land. Stormwater may infiltrate into soil, discharge directly into streams, water bodies, or stormdrains, or evaporate back into the atmosphere.
In the natural environment, most precipitation is absorbed by trees and plants or permeates into the ground, which results in stable stream flows and good water quality.
Things are different in the built environment. Rain that falls on a roof, driveway, patio, or lawn runs off the surface more rapidly, picking up pollutants as it goes. This stormwater runoff flows into streams or storm drains that empty into waterways like the Allegheny, Monongahela, and Ohio rivers.