A water quality standard defines the water quality goals for a water body or portion thereof, in part, by designating the use or uses to be made of the water. The designated beneficial use of a water body must consider its actual use, the ability of the water to support in the future a use that is not currently supported, and the basic goal of the Clean Water Act that all waters support aquatic life and recreation where attainable. Idaho must designate its uses accordingly.

A designated use is a beneficial use assigned to a specific water body in Idaho water quality rules.  The Clean Water Act requires Idaho to recognize existing uses, which are uses that are/were actually attained in a water body on or after November 28, 1975, whether or not they are designated uses. Idaho presumes most undesignated waters will support cold water aquatic life and either primary or secondary contact recreation. These are termed presumed uses. Designated, existing, and presumed uses must all be protected.

In designating uses, Idaho takes into consideration the use and value of the water body for public water supply; for protection of fish, shellfish, and wildlife; and for recreational, agricultural, industrial, and navigational purposes. While there may be competing beneficial uses in a river or a stream, federal law requires DEQ to protect the most sensitive of the beneficial uses.

Idaho evaluates the suitability of a water body for the uses based on the following:

• physical, chemical, and biological characteristics;
• geographical setting and scenic qualities; and
• economic and public values.

  Beneficial Uses Identified in Idaho's Water Quality Standards

The standards associated with this use are designed to protect animal and plant species that live in the water. Some pollutants or conditions that affect aquatic life are water temperature, dissolved oxygen levels, and concentrations of toxic substances such as ammonia, metals, and pesticides. Therefore Idaho's water quality standards set criteria for these pollutants or conditions to protect against adverse effects due to human activities."

The following are subclassifications for the aquatic life designation:

• Bull trout: unique in that this is a species-specific use.
• Cold water: water quality appropriate for the protection and maintenance of a viable aquatic life community for coldwater species.
• Salmonid spawning: waters that provide or could provide a habitat for active self-propagating populations of salmonid fishes.
• Seasonal cold water: water quality appropriate for the protection and maintenance of a viable aquatic life community of cool and coldwater species, where coldwater aquatic life may be absent during, or tolerant of, seasonally warm temperatures.
• Warm water: water quality appropriate for the protection and maintenance of a viable aquatic life community for warm water species.
• Modified: water quality appropriate for an aquatic life community that is limited due to one or more conditions that preclude attainment of reference streams or conditions.

Except for the modified use, the main distinction between the subclassifications of aquatic life is different temperature criteria.

Recreational uses are divided into primary contact and secondary contact recreation. Both of these classifications carry bacteria criteria (IDAPA 58.01.02.251) to protect people from gastrointestinal illness due to incidental ingestion of the water they are recreating in or on. The bacteria levels allowed differ according to the nature of the recreational use and resulting likelihood of unintentionally ingesting water.

• Primary contact recreation applies to waters where people engage in activities that involve immersion in, and likely ingestion of, water, such as swimming, water skiing, and skin diving.
• Secondary contact recreation applies to waters where people engage in activities where ingestion of water may occasionally occur, such as fishing, boating, wading, and infrequent swimming.

Standards associated with this use indicate whether water from a lake or river is suitable for use as a source for a water supply system. Public drinking water is treated before it is delivered to the tap; a separate set of standards governs treated drinking water. Indicators used to measure the safety or usability of surface water bodies as sources for drinking water include turbidity, which may interfere with treatment, and the presence or absence of toxic substances such as metals or pesticides.

The following are subclassifications for the water supply designation:

• Domestic: water quality appropriate for drinking water supplies.
• Agricultural: water quality appropriate for the irrigation of crops or as drinking water for livestock. This use applies to all surface waters of the state.
• Industrial: water quality appropriate for industrial processes. This use applies to all surface waters of the state.

DEQ determines whether a water body fully supports its designated and existing beneficial uses by evaluating whether the applicable water quality standards and criteria are being achieved and whether a healthy, balanced biological community is present. DEQ's Water Body Assessment Guidance describes a process that uses biological and aquatic habitat parameters, as well as traditional water quality data, to assist in the assessment of beneficial use status.

Bioassessment is particularly useful in judging compliance with Idaho's narrative criteria. The following are bio-assessment parameters that DEQ uses to assess the beneficial use status. These are merely parameters and should not be treated as water quality criteria or applied as water quality standards.

• Stream width
• Stream depth
• Stream shade
• Sediment impacts
• Bank stability
• Other physical characteristics of the stream that affect habitat for fish, insects, or other aquatic life

• Distribution and relative abundance of aquatic insects including Ephemeroptera, Plecoptera, and Trichoptera (EPT)
• Level of organic enrichment (as measured using the Hilsenhoff Biotic Index)
• Relative abundance of functional feeding groups of aquatic insects
• Variety and abundance of fish and other aquatic life, such as aquatic plants