Stormwater runoff is rainwater or snowmelt that flows over the land. Runoff can carry sediment and contaminants from streets, rooftops, and lawns to surface water bodies or can infiltrate through the soil to ground water. Storm drains are the metal grates found on urban and suburban streets, often at corners and on the sides of curbs and gutters. They help prevent flooding by draining stormwater runoff that does not percolate into the ground. Storm drains can be independent from sewer systems and may transport untreated stormwater directly to surface water or ground water. As runoff flows over the land or impervious surfaces, it accumulates debris, chemicals, oil, sediment, pet waste, soap used for washing cars, and other activities that can adversely affect water quality.