Protecting Public Health and the Environment.


Federal water quality standards (40 CFR 131.12) require states to develop antidegradation policies to protect existing and designated beneficial uses of surface waters. Idaho's antidegradation policy, contained in Section 051 of Idaho's water quality standards (IDAPA, establishes three tiers of water quality protection.

Tier I

Tier I maintains and protects existing uses and water quality conditions necessary to support such uses. Where an existing use is established, it must be protected even if it is not listed in the water quality standards as a designated use. Tier I requirements are applicable to all surface waters.

Tier II

Tier II maintains and protects "high-quality" waters—water bodies where existing conditions are better than necessary to support Clean Water Act "fishable/swimmable" uses. Water quality may be lowered in Tier II waters, but only with public review of the necessity for degradation and social and economic importance of the activity. In no case may water quality be lowered to a level that would interfere with existing or designated uses.

Tier III

Tier III maintains and protects water quality in outstanding resource waters (ORWs). Except for certain temporary changes, water quality cannot be lowered in such waters. ORWs generally include the highest quality waters of the United States. However, the ORW classification also offers special protection for waters of exceptional ecological significance, such as those that are ecologically important, unique, or sensitive. Decisions regarding which water bodies qualify to be ORWs are made by states and authorized tribes. In Idaho, designation as an ORW requires legislative action. Though several waters in Idaho have been considered for ORW recognition at various times, none have yet been legislatively approved.

Antidegradation Guidance Development

DEQ began developing its antidegradation guidance in 2010. During ensuing months, several public comment opportunities were held on individual chapters and appendices. Based on comments received, the draft document underwent various revisions, culminating in a public comment draft in January 2012. The comment period on the public comment version of the draft guidance closed March 12, 2012.

DEQ has recently completed rulemaking to revise its antidegradation provisions with regard to de minimus discharges. DEQ will submit the provisions to EPA for approval in summer 2014. Once approved, DEQ will work on finalizing its draft guidance.

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Staff Contacts

Water Quality Standards Lead
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0502

DEQ Guidance

Flow Chart for Determining Tier I or Tier II for Aquatic Life and Contact Recreation Beneficial Uses

Draft Guidance Documents

Draft Idaho Antidegradation Implementation Procedures, Version 2
(available for public comment until 1/13/2020)

Related Pages

Water Quality: Docket No. 58-0102-1301 - Final Rule

Water Quality: Docket No. 58-0102-1001 - Final Rule