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TMDL Information

What is a TMDL?

A TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, and an allocation of that amount to the pollutant's sources.

Water quality standards are set by States, Territories, and Tribes. They identify the uses for each waterbody, for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing), and the scientific criteria to support that use.

A TMDL is the sum of the allowable loads of a single pollutant from all contributing point and nonpoint sources. The calculation must include a margin of safety to ensure that the waterbody can be used for the purposes the State has designated. The calculation must also account for seasonal variation in water quality.

The Clean Water Act, section 303, establishes the water quality standards and TMDL programs.

To view current (as of August 2012) TMDLs for Idaho, Utah or Wyoming, click the following links:

What is a HUC?

The United States is divided and sub-divided into successively smaller hydrologic units which are classified into four levels: regions, sub-regions, accounting units, and cataloging units. The hydrologic units are arranged within each other, from the smallest (cataloging units) to the largest (regions). Each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two to eight digits based on the four levels of classification in the hydrologic unit system. For more info, see

How do water bodies get listed?

Under section 303(d) of the 1972 Clean Water Act, states, territories, and authorized tribes are required to develop lists of impaired waters. These impaired waters do not meet water quality standards that states, territories, and authorized tribes have set for them, even after point sources of pollution have installed the minimum required levels of pollution control technology. The law requires that these jurisdictions establish priority rankings for waters on the lists and develop TMDLs for these waters.

What does a TMDL include?

A TMDL will include loads from point sources (such as municipal and industrial wastewater treatment facilities) and nonpoint sources (such as direct runoff from agricultural lands, urban areas, forested lands, etc.) in the watershed. TMDLs will look at impacts on waterbodies from many kinds of contamination, will consider seasonal variations, and will account for background levels of pollutants (those levels occurring in nature). TMDLs compare current loads and proposed loads with expected levels of reduction to achieve the water quality standards.

What does mean if they are delisted?

Delisting a waterbody only means that that the TMDL process was completed.

What is a BMP?

A Best Management Practice (BMP) is usually based on an approach or technology that has been shown to work and to be effective for the purpose intended. The United States Environmental Protection Agency (EPA) uses BMPs to specify standards of practice where a regulation may not be descriptive enough to do so. A BMP should also be as inexpensive as possible and the equipment or technology should be readily available. A BMP may be adopted based on a survey of practices (shooting ranges in this case) that have had successful experiences with an approach or technology. For more information see