A water body is listed as impaired when it fails to meet 1) general federal water quality standards or 2) the specific standards for its designated use (i.e. fishing, swimming or recreational use.) Designated uses are determined by the state.

Once the state determines a river, stream or lake is impaired, it places the water body on its 303(d) list. Waters on this list are divided into those that do not support their designated uses or those that partially support designated uses. When 25 percent or more of the samples collected do not meet the standards for a given pollutant, the river, stream or lake falls in the not supporting category. Waters are listed as partially supporting when 11 to 25 percent of the samples collected do not meet the standard for that use.

The 303(d) list helps the state and federal agencies keep track of what waters are impaired and by what pollutant. Regulatory agencies must develop TMDLs for all the 303(d) listed waters, whether partially supporting or not supporting.

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