The designated use of a water body may be changed if it is shown that it is not attainable. A use attainability analysis (UAA) is a structured scientific assessment of the beneficial uses a water body could support, given application of required effluent limits and implementation of cost-effective and reasonable best management practices.

A UAA is required anytime a state or tribe designates a use that does not include the "fishable/swimmable" goals of the Clean Water Act or changes a use to one that would apply less stringent criteria than the current use. If a use is designated that does not include the "fishable/swimmable" goals, that use designation and the rationale behind it need to be revisited every three years to see if circumstances have changed.

Waters must be protected for the most sensitive of their uses. Most streams have overlapping or competing uses. When this is the case, the most sensitive use is that which requires better water quality (the use with the most stringent criteria). If an existing use is "better" than the designated use, the existing use must be protected and that use must become the designated use. Uses that currently exist, or have existed since November 25, 1975, cannot be removed or downgraded.

The Clean Water Act allows only six reasons for changing a current use designation:

1. Naturally occurring pollutant levels prevent attainment of the use.
2. Natural ephemeral, intermittent, or low flow prevents attainment of the use.
3. Human caused pollution prevents attainment and cannot be remedied without causing worse environmental harm.
4. Dams, diversions, and other hydrologic modifications prevent attainment and it is not feasible to restore the water or operate the modification in a way that would result in attainment.
5. Natural physical features prevent attainment.
6. Controls more stringent than required effluent limitations or new source performance standards would be necessary to attain the use and would result in substantial and widespread social and economic hardship.

Any proposed use change requires rulemaking. In Idaho, rulemaking is at a minimum a several month process that requires public comment and response, Board of Environmental Quality approval, and legislative approval. Use changes are subject to EPA review and approval before being effective for Clean Water Act purposes (discharge permits, total maximum daily loads, §303(d) listing decisions, etc.).

In some instances, the criteria for determining if a designated use is fully supported are not appropriate for a particular water body. A state may reduce the stringency of the criteria for use attainment assigned to a water body by:

1. Revising its water quality criteria (site-specific or statewide). This can only occur where scientific understanding supports the position that a less stringent criterion is still fully protective of the designated uses. This can occur in instances where natural background conditions that support designated uses exceed the water quality criteria.
2. Changing the uses designated for protection under the state standards using the UAA process. If a less sensitive use is established the state can establish less stringent, yet still fully protective, criteria. This can occur if a use is incorrectly assigned to a water body (e.g., the water body is designated for cold water aquatic life, where it should be designated for seasonal cold water).

There is no mechanism to reduce the stringency of criteria established to protect correctly designated or existing uses—except to recognize naturally poor conditions. This is the case when both situations above (1 and 2) do not apply.

Use attainability analysis flow charts show how the process works (pdf 25 kb, 2 pages).

EPA's Technical Justification for UAA Decision, November 29, 2004 (EPA Publication, pdf 312 kb, 59 pages)