Clean Water Act (CWA) 33 U.S.C. §1251 et seq. (1972)
A part of Environmental Law, the CWA uses two methods to protect the quality of water, both monitoring the water quality, and controlling discharge from point sources.
States are required to classify bodies of water by their intended use (swimming, fishing, water supply, navigation, industrial waste disposal), and then must adopt a plans - State Implementation Plan (SIP) to ensure the water meets these ambient water quality standards. Pollution limits must be set to ensure the water is of high enough quality for its designated use. Water quality is controlled by both technology-based standards (requiring polluters to use the best available technology - BAT - to limit pollution, and quality-based standards (by setting Total Maximum Daily Limits - TMDL).
Under §301, discharge of a pollutant from a point source into waters of the United States is illegal without a National Pollution Discharge Elimination System (NPDES) Permit.
- Note that "discharge of a pollutant" is broadly defined such that almost anything can be considered a pollutant (including water with increased sediment that blocks sunlight, or even water at a warmer temperature). However, there are some major exceptions, including agriculture storm water discharge (specifically excluded from the statue) and non point source discharge (such as run-off from agriculture or forestry operations or parking lots)
The scope of the CWA is limited to the "navigable waters" of the united states, but this has been broadly defined to include wetlands, or areas that are underwater at times, but dry at other times, as well as areas directly adjacent to navigable waters. This has been limited to not include unconnected ponds (as created by an old mining quarry).