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American Falls Subbasin

Data Details
Hydrologic Unit Code 17040206
Size 2,869 square miles (1.8 million acres)
Beneficial Uses Affected 607 square miles (388,587 acres)
Major Land Uses Range, forest, agriculture, recreation
Date Approved by EPA Subbasin assessment only; not subject to EPA approval

Subbasin Characteristics

The American Falls subbasin covers 2,869 square miles in southeastern Idaho—much of which is within the Fort Hall Indian Reservation. American Falls Reservoir is the predominant water body in the subbasin and provides both irrigation water and electricity.

2012 Subbasin Assessment and TMDL

TMDLs were developed for nutrients on 17 water bodies in the subbasin and for sediment on 9 water bodies. TMDLs were also developed for nutrients and sediment for six point sources of pollution. Two of the point source wasteload allocations (sediment from the city of Blackfoot’s storm water runoff and nutrients from the Aberdeen wastewater treatment plant) require a reduction from current estimated wasteloads.

American Falls Reservoir and McTucker Creek are § 303(d) listed for flow alteration. However, the US Environmental Protection Agency does not believe that flow alteration is a pollutant as defined by the Clean Water Act. Since TMDLs are not required for water bodies impaired by pollution but not pollutants, TMDLs were not developed for flow alteration.

TMDLs were also not established for dissolved oxygen or bacteria. Dissolved oxygen was listed for the Snake River and American Falls Reservoir. Data did not indicate dissolved oxygen was a problem in the Snake River, and it was assumed that a nutrient TMDL will bring dissolved oxygen levels in the reservoir into compliance with water quality standards. Insufficient data existed to determine if bacteria impair recreation in Bannock Creek, so more data will be collected to determine beneficial use support.

The July 2006 analysis recommended that American Falls Reservoir be delisted for sediment and the Snake River be delisted for dissolved oxygen and nutrients. It is anticipated that American Falls Reservoir and the Snake River will be listed for temperature on future § 303(d) lists. The document also recommended that Danielson Creek, Hazard Creek (Little Hole Draw), and Sunbeam Creek be added to future § 303(d) lists.

In March 2009, the document was revised as a requirement of a contested case settlement agreement between J.R. Simplot, the City of Pocatello, and DEQ. Changes to the plan include the following:

  • Removal of nitrogen allocations for tributaries and point sources
  • Adoption of a tiered phosphorus target to achieve water quality standards and support beneficial uses with an interim phosphorus target of 0.07 mg/L total phosphorus
  • Ability to allow water pollutant trading and develop a trading framework if pollutant trading is deemed viable in the watershed

Additionally, in 2012, total phosphorus wasteload allocations for two conservation fish hatcheries (Idaho Department of Fish and Game’s Springfield Hatchery and the Shoshone-Bannock Tribes’ Crystal Springs Hatchery) were included based on a placeholder allocation remaining from historical operations at the Crystal Springs aquaculture facilities near Springfield. These hatchery operations are an important component to the State of Idaho and tribes’ efforts to restore endangered Sockeye and Chinook Salmon in central Idaho.

2012 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Streams Pollutants
American Falls Reservoir Nutrients (total phosphorus)
Snake River Nutrients (total phosphorus), sediment
Bannock Creek (includes Knox Creek) Nutrients (total phosphorus), sediment
Moonshine Creek Sediment
Rattlesnake Creek Sediment
West Fork Bannock Creek Sediment
McTucker Creek Nutrients (total phosphorus), sediment
Danielson Creek Nutrients (total phosphorus), sediment
Hazard Creek (Little Hole Draw) Nutrients (total phosphorus), sediment
Sunbeam Creek Nutrients (total phosphorus), sediment
Clear Creek Nutrients (total phosphorus)
Seagull Bay Tributary Nutrients (total phosphorus)
Spring Creek Nutrients (total phosphorus)
Cedar Spillway Nutrients (total phosphorus)
Colburn Wasteway Nutrients (total phosphorus)
Crystal Springs Nutrients (total phosphorus)
Nash Spill Nutrients (total phosphorus)
R Spill Nutrients (total phosphorus)
Spring Hollow Nutrients (total phosphorus)
Sterling Wasteway Nutrients (total phosphorus)
Portneuf River (on the § 303(d) list for its own subbasin; Portneuf River TMDL already approved) Nutrients (total phosphorus)

Subbasin Documents

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DEQ Pocatello Regional Office
444 Hospital Way, #300
Pocatello, ID 83201
Ph: (208) 236-6160
Fax: (208) 236-6168
Toll Free: (888) 655-6160

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