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Willow Creek Subbasin

Data Details
Hydrologic Unit Code 17040205
Size 693 square miles (443,458 acres)
Water Bodies with EPA-Approved TMDLs (Category 4a) Brockman Creek, Corral Creek, Crane Creek, Grays Lake outlet, Hell Creek, Homer Creek, Lava Creek, Meadow Creek, Mill Creek, Sawmill Creek, Sellars Creek, Tex Creek, Willow Creek
Beneficial Uses Affected Cold water aquatic life, salmonid spawning, primary contact recreation, secondary contact recreation, domestic water supply
Major Land Uses
Cropland, rangeland, forest, water (Grays Lake)
Date Approved by EPA June 2004 Approval Letter

Subbasin Characteristics

The Willow Creek subbasin in southeastern Idaho is a watershed of the Upper Snake River basin. Waters of Willow Creek are connected to the Snake River through a complex irrigation system located below Ririe Reservoir.

Native fish populations, water quality, and riparian habitat conditions are issues of concern in the subbasin. The cumulative effects of land management in riparian areas, human-caused stream alterations, roads, limited recreation, and pockets of timber harvesting have combined to limit compliance with water quality standards. The production and survival of resident fishes are also affected throughout the watershed.

Rainbow Trout, Yellowstone Cutthroat Trout, Brook Trout, and Brown Trout have all been documented in the watershed. The Yellowstone Cutthroat Trout is a state sensitive species. Fish count data show that salmonid populations are trending downwards in the subbasin.

2004 Subbasin Assessment and TMDL

The document sets TMDLs to control pollution from sediment and to lower temperatures in various segments of the subbasin. In addition, it recommends certain adjustments to the state’s list of impaired water bodies to reflect current conditions.

The magnitude of sediment loading within the subbasin is widespread, predominantly attributable to streambank erosion from overutilization of riparian habitat. Some additional sources of sediment loading are poor road maintenance, road crossings, and mass wasting. Sediment loading targets were developed based on literature detailing expected natural conditions and substrate sediment impacts on salmonid spawning.

Reduced riparian vegetation contributes to accelerated streambank erosion, resulting in increased thermal loading, which, combined with associated changes in channel morphology, is the primary cause of increased temperature loading in affected streams. Temperature TMDLs have been developed for all streams where thermograph data have been collected.

Anthropogenic causes of flow alteration in the subbasin include diversion for stock watering and irrigation. It is not likely that beneficial uses will be restored in streams of the watershed where dewatering from surface water diversions occurs during significant portions of the year. In addition, the US Environmental Protection Agency does not believe that flow (or lack of flow) 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-altered streams.

2004 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Stream Pollutants
Brockman Creek Sediment, temperature
Buck Creek Sediment
Corral Creek Sediment, temperature
Crane Creek Sediment
Grays Lake Outlet Sediment
Hell Creek Sediment, temperature
Homer Creek Sediment, temperature
Lava Creek Sediment, temperature
Meadow Creek Sediment
Mill Creek Sediment, temperature
Rock Creek Sediment
Sawmill Creek Sediment, temperature
Sellars Creek Sediment, temperature
Seventy Creek Sediment
Tex Creek Sediment, temperature
Willow Creek Sediment, temperature

Subbasin Document(s)

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DEQ Idaho Falls Regional Office
900 N. Skyline Drive, Suite B
Idaho Falls, ID 83402
Ph: (208) 528-2650
Fax: (208) 528-2695
Toll Free: (800) 232-4635

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