The EPA’s “National Pollutant Discharge Elimination System (NPDES) Storm Water Program Questions and Answers” document states:

Polluted stormwater runoff is a leading cause of impairment to the nearly 40 percent of surveyed U.S. water bodies which do not meet water quality standards. Over land or via storm sewer systems, polluted runoff is discharged, often untreated, directly into local water bodies. When left uncontrolled, this water pollution can result in the destruction of fish, wildlife and aquatic life habitats; a loss in aesthetic value; and threats to public health due to contaminated food, drinking water supplies; and recreational waterways.

Mandated by Congress under the Clean Water Act, the NPDES stormwater program is a comprehensive two-phased national program for addressing the non-agricultural sources of stormwater discharges which adversely affect the quality of our nation’s waters. The program uses the NPDES permitting mechanism to require the implementation of controls designed to prevent harmful pollutants from being washed by stormwater runoff into local water bodies.

View more information on the NPDES.