Protecting Public Health and the Environment.

Water Quality Standards

Water quality standards are the benchmarks DEQ uses to know if it is doing its job to protect 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 serve the purposes of the Clean Water Act, which states that water quality standards should:

  • provide, wherever attainable, water quality for the protection and propagation of fish, shellfish, and wildlife, and recreation in and on the water (fishable/swimmable conditions); and
  • consider the use and value of state waters for public water supplies, propagation of fish and wildlife, recreation, agriculture and industrial purposes, and navigation.

The Idaho water quality standards program is a joint effort between DEQ and EPA. DEQ is responsible for developing and enforcing water quality standards that protect beneficial uses such as drinking water, cold water fisheries, industrial water supply, recreation, and agricultural water supply. EPA develops regulations, policies, and guidance to help Idaho implement the 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, where necessary, to promulgate federal water quality rules (such as the Idaho Bull Trout Rule).

Elements of a Water Quality Standard

A water quality standard defines the designated beneficial uses of a water segment and the water quality criteria necessary to support those uses. Water quality standards are important because they help to protect and restore the quality of the Idaho's surface waters. Associated 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).

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

  • Antidegradation: an antidegradation policy to maintain and protect existing uses and high quality waters.
  • Beneficial Uses: designated uses of the water body (e.g., recreation, water supply, aquatic life, agriculture),
  • Criteria: water quality criteria to protect designated uses (numeric pollutant concentrations and narrative requirements), and

The state also has the prerogative to create general policies that address implementation issues such as low flows, variances, and mixing zones.

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 exist when there is no measurable difference between the quality of water now and the quality of water that would exist if there were no human-caused changes in the watershed. Idaho is among the states that allows water quality to exceed numeric criteria due to natural background conditions of the water body. Learn more.

Mixing Zones

A mixing zone is a defined area or volume of a receiving water surrounding by or adjacent to a wastewater discharge where the receiving water, as a result of the discharge, may not meet all applicable water quality criteria standards. The use of mixing zones is allowed in Idaho 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 based upon 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 Proposed Standards

Criteria adopted by Idaho for temperature are under review by EPA; certain toxics 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

Water Quality Standards Coordinator
Don Essig
(208) 373-0119

Water Quality Standards Scientist
Mary Anne Nelson
(208) 373-0173

Water Quality Standards Analyst
Miranda Adams
(208) 373-0574

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)