Water moves through nature in cycles. Heat from sunlight causes the surface waters of oceans and lakes to evaporate and form clouds. Water also moves through vegetation and transpires to the atmosphere.
Water returns to Earth as rain or snow.
More often, the conveyances discharge the stormwater, at discrete points commonly called outfalls, directly into streams, rivers, coastal waters, and the ocean. Pollutants carried by stormwater contribute to water quality and habitat problems affecting our nation's coasts. Urbanization and increasing population growth exacerbate stormwater pollution problems in two critical ways. First, increasing populations generate more contaminants. Second, to accommodate expanding populations, communities radically alter land uses by replacing wetlands, forests, and other natural areas with impervious surfaces, such as roads and buildings.