Stormwater runoff is the result of impervious surfaces (surfaces that cannot absorb rain water -- asphalt, concrete, and rooftops) replacing pervious surfaces (surfaces that can absorb and filter rain water -- forests and prairies). As urban populations grow, impervious surfaces are rapidly replacing pervious surfaces.

When rain falls on impervious surfaces, the water collects until it "runs off" to the point of lowest elevation, which is usually a waterway. This is referred to as stormwater runoff.