• Shellfish harvest: The water body supports a population of shellfish free from toxicants and pathogens that could pose a human health risk to consumers.

  • Drinking water supply: The water body can supply safe drinking water with conventional treatment.

  • Primary contact recreation or swimming: People can swim in the water body without risk of adverse human health effects.

  • Secondary contact recreation: People can perform activities on the water (such as canoeing) without risking adverse human health effects from occasional contact with the water.

  • Agriculture: The water quality is suitable for irrigating fields or watering livestock.

Regarding these beneficial uses, there are five levels of use support recognized by the EPA:

  1. Fully supporting overall use: All designated beneficial uses are supported fully.

  2. Threatened overall use: One or more of the designated beneficial uses are threatened and the remaining uses are supported fully.

  3. Partially supporting overall use: One or more of the designated beneficial uses are partially supported and the remaining uses are supported fully.

  4. Not supporting overall use: One or more designated beneficial uses are not supported.

  5. Not attainable: The state has performed a use-attainability study and documented that support of one or more designated beneficial uses is not achievable.

The EPA defines impaired waters as the sum of water bodies partially supporting uses and not supporting uses.

The data on rivers, lakes, and wetlands below are from 1992 state assessments under section 305(b) of the Clean Water Act as summarized by EPA (1994) for use in the Congress.


The United States has approximately 3.5 million miles of rivers and streams. Of the 642,881 miles of rivers assessed by the states in 1992, more than one-third did not fully support designated uses. The five leading sources of river water quality impairment listed by the states were (1) agriculture, (2) municipal point sources, (3) urban runoff or storm sewers, (4) resource extraction, and (5) organic enrichment or low dissolved oxygen. Forty-five states identified almost 160,000 river miles impaired by agricultural sources.

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