Protecting Public Health and the Environment.

Water Quality Standards

Water quality standards are the benchmarks DEQ uses to know if it is adequately protecting Idaho's surface water—streams, rivers, lakes, and reservoirs. Idaho adopts water quality standards (see IDAPA 58.01.02) to protect public health and welfare, enhance the quality of water, and meet requirements of the Clean Water Act, which states that water quality standards should do the following:

  • Provide water quality for the protection and propagation of fish, shellfish, and wildlife and recreation in and on the water (fishable/swimmable conditions), where attainable.
  • Consider the use and value of state waters for public water supplies, propagation of fish and wildlife, recreation, agricultural and industrial purposes, and navigation.

The Idaho Water Quality Standards Program is a joint effort between DEQ and EPA. DEQ is responsible for adopting and enforcing water quality standards that protect beneficial uses. EPA develops recommended criteria, regulations, policies, and guidance to help Idaho implement the Water Quality Standards Program and to ensure that Idaho's adopted standards are consistent with the requirements of the Clean Water Act and relevant regulations. EPA has authority to review and approve or disapprove state standards and to promulgate federal water quality rules if it finds the state is not meeting the requirements of the Clean Water Act (such as the Idaho Bull Trout Rule).

Elements of Water Quality Standards

Water quality standards define the designated beneficial uses of a water segment and the quality of the water (i.e., criteria) necessary to support those uses. Water quality standards are important because they let us know whether or not we are sufficiently protecting the quality of Idaho's surface waters and provide the goals when that quality must be restored. Criteria may be numeric (i.e., not to exceed some concentration) or narrative. Narrative criteria are sometimes referred to as "free from" criteria, as they often state that the water body must be free from something (e.g., free from nuisance aquatic growths).

The water quality standard consists of three basic elements (the "ABCs" of water quality):

  • Antidegradation: a policy to maintain and protect existing uses and high quality waters
  • Beneficial Uses: uses of the water body (e.g., recreation, water supply, aquatic life, agriculture)
  • Criteria: the level of water quality needed to protect beneficial uses (numeric concentrations and narrative requirements)

The state also has the prerogative to create general policies that address implementation issues, such as allowing variances and mixing zones and specifying unusual conditions (e.g., lows flows) during which criteria are allowed to be exceeded.

Triennial Review

Section 303 of the Clean Water Act (CWA) requires states to modify and improve their water quality standards (WQS) at least once every three years. Under this triennial review process, states are to review, and modify and adopt as appropriate, applicable water quality standards, taking into consideration public concerns, EPA guidance, and new scientific and technical information. Learn about how DEQ intends to conduct its triennial review in 2014-16 here.

Natural Background Conditions

Natural background conditions describe the quality of water that would exist without human-caused changes in the watershed. In some settings, this natural water quality may be poorer than the criteria set to protect uses. The Idaho water quality standards acknowledge this and allow water quality to naturally exceed criteria. Learn more.

Mixing Zones

A mixing zone is a limited and defined area or volume of a receiving water body adjacent to a wastewater discharge where the receiving water, as a result of the discharge, may not meet all applicable water quality criteria. Mixing zones may be granted by DEQ under certain conditions. Learn more.

Variances from Idaho Water Quality Standards

A variance is a temporary relaxation of water quality standards. Variances are granted by DEQ to facilities for specified pollutants in their wastewater discharge based on a rationale as to why more time is needed to meet the prevailing criteria. Three variances have been granted in Idaho to date. Learn more.

§401 Certification

Section 401 of the federal Clean Water Act requires state certification for any permit or license issued by a federal agency for an activity that may result in a discharge into waters of the U.S., thereby ensuring state input into federally approved projects that may affect its waters. Learn more.

EPA Actions on Idaho Water Quality Standards

Water quality standards adopted by Idaho are not valid for Clean Water Act purposes such as permitting or TMDLs until approved by EPA. Some Idaho criteria are still under review by EPA; other criteria have been disapproved by EPA. Learn more.

DEQ State Office - Water Quality Division

1410 N. Hilton
Boise, ID 83706
(208) 373-0502

Staff Contacts

Senior Water Quality Standards Scientist
Michelle Dale
(208) 373-0187

401 Program Lead

DEQ Resources

Spreadsheets to easily calculate criteria values given hardness, pH, or temperature for:

EPA Guidance

Water Quality Standards Handbook: Second Edition (March 2012)
The WQS Handbook is a compilation of EPA's guidance on the water quality standards program and provides direction for states in reviewing, revising and implementing water quality standards.

More Information

Idaho's Water Quality Standards (IDAPA 58.01.02)