Beneficial Use Support
DEQ determines whether a water body fully supports its 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 assessing beneficial use status. Bioassessment is particularly useful in judging compliance with Idaho's narrative criteria. The following are bioassessment parameters that DEQ uses to assess beneficial use status. These are merely parameters and should not be treated as water quality criteria or applied as water quality standards.
Aquatic Habitat Parameters
- 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
Natural Background Conditions
Although numeric water quality criteria are carefully set, it is impossible to pick a single broadly applied value for some measures (e.g., temperature and dissolved oxygen) that is never exceeded naturally, yet is protective of uses. Therefore, Idaho water quality standards allow that if natural background conditions exceed any applicable water quality criteria, no impairment of beneficial uses or violation of water quality standards exists. Natural background conditions alone shall not be the basis for placing a water body on the list of water quality limited water bodies.