The Clean Water Act, adopted in 1972, established a regulatory process called the National Pollutant Discharge Elimination System (NPDES). The Act sought to clean up the nation's polluted surface waters with the goal of making them "swimmable and fishable." The U.S. Environmental Protection Agency (EPA) recognized that many of the nation's surface waters were still highly impaired due to the continued discharge of "non-point" source pollutants carried by stormwater runoff. Amendments to the Clean Water Act, adopted in 1987, established specific requirements that regulated stormwater discharges from Municipal Separate Storm Sewer Systems (MS4s).
Stormwater NPDES permitting requirements were proposed to be implemented in two phases. Phase I covered municipalities with populations greater than 100,000 with permits issued during the early 1990s. In 1999, Phase II was instituted by the EPA to cover municipalities with populations less than 100,000 and small MS4s in urbanized areas. It took effect on March 10, 2003. The NPDES program requires permits to develop and implement a stormwater management program that addresses the six minimum control measures.