Department of
Environmental Quality

Subbasin at a Glance

Overview

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.

TMDLs were developed for nutrients on 17 water bodies in the subbasin and for sediment on nine water bodies. TMDLs were also developed 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 U.S. 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 Snake River, and it was assumed that a nutrient TMDL will bring dissolved oxygen levels in the reservoir into compliance with water quality standards. There were insufficient data 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:

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

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

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

• American Falls Subbasin Total Maximum Daily Load Plan: Subbasin Assessment and Loading Analysis (May 2012)