The Clean Water Act (CWA) is designed to "protect and maintain" the "chemical, physical[,] and biological integrity" of the nation's waters. How to get there? Three steps. First, establish water quality standards (standards) to make sure each state water body is safe and clean enough to use for its common purpose, such as drinking, fishing, swimming, and boating. Second, require point sources to implement categorical engineering solutions to reduce pollutants in effluent discharge. Third, where these solutions are not sufficient to ensure compliance with standards for receiving water, use site-specific ones. How? Determine how much pollution a water body can assimilate before it no longer meets standards. Calculated day to day across the seasons and allowing for a margin of safety, we call this a total maximum daily load (TMDL). Then apportion the TMDL among sources, both point and nonpoint. Results? Water safe and clean enough for common uses such as fishing, swimming, boating, drinking, and commerce.

Given the limits of technology-based approaches, TMDLs have become the CWA's linchpin. The U.S. Congress intended for states and the U.S. Environmental Protection Agency (EPA) to set TMDLs beginning in 1974. It didn't happen. EPA promulgated rules in 1979, but thereafter, still no luck. In 1984, a federal court of appeals found a state's "prolonged failure" to set a TMDL for an impaired water amounted to a "constructive submission" of no TMDL, which EPA was duty bound to approve or disapprove. Seeking clarity and programmatic relevance, EPA amended its TMDL rules in 1985, and again in 1992.