Runoff

Surface runoff occurs when soil is fully saturated, and excess water from rain--or other sources--flows over the land instead of draining into it. Development and urbanization increase runoff in areas like Los Angeles, thanks to pavement and buildings, which reduce the amount of natural soil at the surface available for rainwater. Although a small amount of runoff is natural, the larger amounts produced in urban areas often lead to erosion, flooding, and water pollution, as excess water often picks up contaminants as it flows to larger water sources like rivers or watersheds.