Water Quality Criteria
Idaho's water quality standards prescribe certain criteria that must be met to ensure the beneficial uses of the state's surface waters are supported. These criteria can be numeric or narrative. Numeric criteria are use-specific, while narrative criteria are general, applying to all waters regardless of use. Federal water quality standard regulations require that together, the numeric and narrative criteria must contain sufficient parameters or constituents to protect the beneficial use.
Common Water Quality Measures
Some of the more common measures of water quality are temperature, turbidity, bacteria, dissolved oxygen, and pH. Less common are measurements of sediment and nutrients. Rarely, or in limited locations, measurements of toxic substances such as metals, pesticides, and ammonia are available. All are important in influencing the health or utility of a water body. Learn more.
Numeric Water Quality Criteria
Individual numeric criteria are based on specific data and scientific assessment of adverse effects. Numeric guidelines assign numbers that represent limits and/or ranges of chemical concentrations, like dissolved oxygen, or physical conditions, like water temperature. Learn more.
Narrative Water Quality Criteria
To supplement numeric criteria, Idaho has adopted narrative criteria, which are statements that describe the desired water quality goal, such as Idaho's waters being "free from" pollutants. These "fuzzy" criteria require much case-by-case interpretation but are important in bridging gaps in knowledge. Learn more.
Site-Specific Water Quality Criteria
After site-specific analyses in some water bodies, it may be prudent to develop alternative criteria. Site-specific criteria may be developed to more appropriately regulate sources of pollution while still protecting designated and existing beneficial uses. These new criteria would then be applicable only to the segment for which they were developed. Learn more.
Toxic Substance Criteria
A toxic substance is any substance, material, or disease-causing agent that, upon exposure, ingestion, inhalation, or assimilation into an organism will cause death, disease, malignancy, physical deformations, or other abnormalities in affected organisms or their offspring. Toxic criteria exist to protect both aquatic life and human health. Because aquatic life and human health differ in sensitivity to various toxins, the parameters for which criteria are set, as well as their levels, usually differ as well. Learn more.