The federal Clean Water Act requires states and tribes to restore and maintain the chemical, physical, and biological integrity of the nation's waters and to adopt water quality standards necessary to protect fish, shellfish, and wildlife while providing for recreation in and on the waters whenever possible.
Water quality standards have been established by the Idaho legislature and approved by the US Environmental Protection Agency (EPA). These standards are designed to protect, restore, and preserve water quality in areas designated for specific uses such as cold-, cool-, or warm-water fisheries; agricultural water supply; recreation; wildlife habitat; and aesthetics. Uses have been designated for most, but not all, water bodies within Idaho.
Section 303(d) of the Clean Water Act establishes requirements for states and tribes to identify and prioritize water bodies that do not meet water quality standards. Currently, DEQ is required to conduct a comprehensive analysis of Idaho's water bodies every two years to determine if they meet water quality standards. This analysis is published and submitted to the EPA in Idaho's Integrated Report. Idaho must develop a water quality improvement plan, called a total maximum daily load (TMDL), for those water bodies not meeting water quality standards. A subbasin assessment (SBA) is the first step toward developing a TMDL.
About 36% of Idaho's streams were identified in the 2010 Integrated Report as not meeting water quality standards, and TMDLs must be developed for each of these. Idaho and EPA have a legal, court-ordered responsibility to ensure that these impaired waters are dealt with in a timely manner (see TMDL settlement agreement and schedule).
In Idaho, TMDLs are assessed on a subbasin level, which means water bodies and pollutants within a hydrologic subbasin are generally addressed in a single document. A subbasin is based on a cataloging unit of the US Geological Survey (USGS) (i.e., a subbasin is the same as a USGS fourth-field hydrologic unit code, or HUC). Idaho has 86 subbasins (HUCs) entirely or partially within its borders.
Although the map above shows portions of 86 HUCs, 2 of those—17010103 (Yaak) and 17060107 (Lower Snake)—have no waters within the state of Idaho.
Interested in finding out whether a subbasin assessment has been conducted and a TMDL developed for a specific water body in Idaho? Find out here.
An implementation plan is a document guided by an approved TMDL that provides details of the actions needed to achieve load reductions, outlines a schedule of those actions, and specifies monitoring needed to document action and progress toward meeting water quality standards. Learn more.
Basin advisory groups (BAGs) are groups of citizens that advise DEQ on water quality objectives within Idaho's six basins. Its members are appointed by the director of DEQ and represent a cross section of interests in the basin, such as those representing agriculture, forestry, municipalities, industry, recreation, Native American tribes, and environmental concerns. Learn more.
Watershed advisory groups (WAGs) are groups of citizens, appointed by DEQ's director, that provide guidance on specific watersheds within Idaho's six basins. WAGs provide local public input and guidance to DEQ when developing a TMDL. Members of each WAG represent the industries and interests affected by the management of that watershed. The WAG also includes representatives of local government and the land management or regulatory agencies interested in the management of water quality in the watershed. Learn more.
What is a subbasin assessment? Why does DEQ conduct subbasin assessments? What steps are involved in TMDL development? Learn the answers to these and more questions here.
Many of the issues involved in developing a TMDL are technical or legal in nature. Technical advisory groups, or TAGs, can assist the WAG in evaluating these issues. TAGs are comprised of knowledgeable citizens and experts from groups like DEQ, the Idaho Department of Fish and Game, the US Department of Agriculture, the US Forest Service, Native American tribes, EPA, and other groups or agencies, depending on the issues at hand.