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Brownlee Reservoir Subbasin

Data Details
Hydrologic Unit Codes 17050201
Size 156 square miles (99,806 acres)
Water Bodies with EPA-Approved TMDLs (Category 4a) Bear Creek, Deer Creek, Dennett Creek, Hog Creek, Jenkins Creek watershed, Lick Creek, Scott Creek, Snake River-Hells Canyon Reservoir, Snake River-Oxbow Reservoir, Upper and Lower Brownlee Reservoir, Warm Springs Creek, Wildhorse River
Beneficial Uses Affected Cold water aquatic life, salmonid spawning, primary and secondary contact recreation, agricultural and industrial water supply, wildlife habitat, aesthetics
Major Land Uses Agriculture, forest, urban/suburban, livestock grazing, recreation
Approved by EPA September 2003 EPA Approval Letter
Wildhorse River TMDL Approved by EPA October 2007 EPA Approval Letter
Weiser Flats TMDL Approved by EPA July 2021 EPA Approval Letter

Subbasin Characteristics

The Brownlee Reservoir subbasin encompasses the area draining into the Snake River downstream of the Weiser River inflow and upstream of Brownlee Reservoir. This subbasin is located along the central portion of the Idaho-Oregon border in southwestern Idaho. The headwaters for these creeks originate in extreme western Idaho in the Hitt Mountains.

2003 Weiser Flat Subbasin Assessment and TMDL

Five water quality limited streams exist in the subbasin, four of which were placed on Idaho’s 1998 § 303(d) list and one of which (Jenkins Creek) was removed from the list in 1998. However, recent monitoring has identified water quality concerns below the original assessment site that necessitate relisting Jenkins Creek.

There are clear indications that recreational beneficial uses are not fully supported for the Hog, Scott, Warm Springs, and Jenkins Creeks subwatersheds. However, the available information for cold water aquatic life uses is limited. Support of cold water aquatic life uses cannot be determined specific to nutrient and sediment concentrations due to a lack of aquatic life data. Interpretation of data available for nutrients and sediments is tied to downstream waters (Snake River-Hells Canyon TMDL) shown to be impaired due to excess nutrient and sediment loading. Secondary contact recreation is not supported in Hog, Scott, Warm Springs, or Jenkins Creeks due to exceedances of bacteria standards during the spring and summer months. All listed water bodies exhibit monitored concentrations of total phosphorus above the target identified to improve water quality in the Hells Canyon reach of the Snake River.

Bacteria is proposed to be listed for Hog, Scott, Warm Springs, and Jenkins Creeks as a § 303(d) pollutant as part of the first § 303(d) list submitted by the State of Idaho subsequent to the approval of this TMDL.

2003 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Stream Pollutants
Dennett Creek Sediment
Hog Creek Phosphorus
Scott Creek Phosphorus, sediment
Warm Springs Creek Phosphorus, sediment
Jenkins Creek Phosphorus, sediment

2007 Wildhorse River Watershed Assessment and TMDL

The Wildhorse River watershed is part of the Brownlee Reservoir subbasin, which is located in southwestern Idaho on the border between Idaho and Oregon.

The headwaters of the Wildhorse River originate in forested land at the southern end of the Seven Devils Mountains, which form the eastern border of Hells Canyon. The river flows southwesterly out of these mountains and enters the Snake River between Brownlee Dam and Oxbow Reservoir. This portion of the Snake River forms the border between Idaho and Oregon. Although some of the southerly tributaries flow out of Washington County, the main stem is located solely in the southern portion of Adams County. No towns and very few inhabitants are located on the Wildhorse River. The unincorporated community of Bear is located in the Wildhorse River subbasin.

In 2000, the US Environmental Protection Agency (EPA) added streams to Idaho’s 1998 § 303(d) list of impaired waters that exceeded Idaho’s temperature criteria. In the Brownlee Reservoir subbasin, Wildhorse River was among those EPA additions.

To fully evaluate the heat loading to this river, its major tributaries (Bear Creek, Lick Creek, and Crooked River) were also examined.

Effective shade targets were established for Wildhorse River and its three major tributaries. An analysis of shade revealed that the Wildhorse River is slightly below target shade levels and requires a 12% reduction in solar load to achieve background conditions. Additionally, the heat loading in the river is compounded by the excess solar loading to Lick Creek, Bear Creek, and Crooked River. Most streams appear to be in relatively good condition and should be considered relatively low priority for implementation.

2007 TMDL: Stream and Pollutant for Which TMDLs Were Developed

Stream Pollutants
Wildhorse River Temperature

2021 Weiser Flat E. coli TMDL

This TMDL addresses five water bodies in the Brownlee subbasin that are listed in Category 5 of Idaho’s most recent Integrated Report. This document established bacteria (E. coli) TMDLs for seven assessment units that are impacted by non-point source pollution. The TMDL analysis establishes water quality targets, load capacities, estimates of existing pollutant loads, and allocations load reductions needed to return the listed waters to a condition meeting water quality standards.

Stream Pollutants
JenkinsCreek E. coli
Scott Creek E. coli
Warm Springs Creek E. coli
Hog Creek E. coli
Rock Creek E. coli

Subbasin Documents

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1445 N. Orchard St.
Boise, ID 83706
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