Protecting Public Health and the Environment.

American Falls Subbasin

Subbasin at a Glance

Hydrologic Unit Code 17040206
Size 2,869 square miles (1.8 million acres)

Water Bodies with EPA-Approved TMDLs (Category 4a)

American Falls Reservoir and tributaries, Bannock Creek, Clear Creek, Danielson Creek, Knox Creek, Little Hole Draw, McTucker Creek, Moonshine Creek, Pleasant Valley Creek, Rattlesnake Creek, Snake River, Spring Creek, Sunbeam Creek, West Fork Bannock Creek

Beneficial Uses Affected Cold water aquatic life, salmonid spawning, primary and secondary contact recreation, domestic water supply, aesthetics

Major Land Uses

Agriculture, grazing, urban

Date Approved by EPA

August 2012
EPA Approval Letter

Subbasin Characteristics

The American Falls subbasin covers 2,869 square miles in southeast 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 stormwater 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:

  1. Removal of nitrogen allocations for tributaries and point sources
  2. 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
  3. 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

American Falls Reservoir
Nutrients (total phosphorus)
Snake River
Nutrients (total phosphorus), sediment
Bannock Creek (includes Knox Creek)
Nutrients (total phosphorus), sediment
Moonshine Creek
Rattlesnake Creek
West Fork Bannock Creek
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 Document

DEQ Pocatello Regional Office

444 Hospital Way #300
Pocatello, ID 83201
(208) 236-6160
toll-free: (888) 655-6160

Staff Contacts

Surface Water Quality Manager
Lynn Van Every
(208) 236-6160

Related Pages

Frequently Asked Questions about Subbasin Assessments and TMDLs

TMDL Implementation Plans

Table of Subbasin Assessments, TMDLs, Implementation Plans, and Five-Year Reviews