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http://www.epa.gov/region6/water/npdes/tmdl/definitions.htm Section 303(d) of the 1972 Clean Water Act (CWA) requires all states to develop a list of their state’s impaired waterbodies. The 303(d) list of impaired waterbodies consists of those waterbodies that do not meet state regulatory water quality standards even with the current pollution controls in place (even after point sources of pollution have installed the minimum levels of pollution controls). The CWA also requires all states to establish priority rankings for waters on the 303(d) list and develop
Total Maximum Daily Loads (TMDLs) for these waters based on their individual priority ranking.
A TMDL is a pollution budget for a specific waterbody (river, lake, stream, etc.). It is the maximum amount of a pollutant (sum of allowable pollutant loads from point and nonpoint sources) that can be released into a waterbody without causing the waterbody to become impaired and/or violate state water quality standards. A TMDL also must include a margin of safety to allow for any uncertainties in the scientific methods used to derive the TMDL (water quality modeling assumptions, etc.).