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Pahsimeroi River Subbasin

Data Details
Hydrologic Unit Codes 17060202
Size 839 square miles (536,960 acres)
Water Bodies with EPA-Approved TMDLs (Category 4a) East Fork Pahsimeroi River, North Fork Lawson Creek, Pahsimeroi River, Short Creek
Beneficial Uses Affected Domestic water supply, cold water aquatic life, salmonid spawning, primary contact recreation
Major Land Uses rrigated agriculture, dryland agriculture, rangeland, forest
Date Approved by EPA December 2001 EPA Approval Letter
Addendum Approved by EPA April 2014 EPA Approval Letter

Subbasin Characteristics

The Pahsimeroi River subbasin is located in east-central Idaho between the Lost River Mountain Range and the Lemhi Mountain Range. The Pahsimeroi River originates near the highest peak in Idaho, Borah Peak, within the Lost River Range. The river flows northward and joins the Salmon River near the town-site of Ellis. The Pahsimeroi River subbasin is somewhat unique in Idaho in that streams from the mountains disappear into the gravel-filled valley and feed the base flow of the Pahsimeroi River from primarily subsurface flow.

The drainage area of the Pahsimeroi River is approximately 840 square miles. Elevations of the valley floor vary from 7,800 feet near the divide with the Little Lost River drainage to 4,600 feet at the confluence with the Salmon River. Elevations in surrounding mountains are as high as 10,971 feet in the Lemhi Mountains, 12,662 feet (Borah Peak) in the Lost River Mountains, and 9,550 feet in the Donkey Hills, which separate the drainage from the Little Lost River.

2001 Subbasin Assessment and TMDL

Water quality, native fish populations, and riparian habitat conditions have been issues of concern in the Pahsimeroi River watershed since the combined effects of warm season grazing, exotic species, and human-caused diversion have synergized with the natural infiltration of streamflows to limit the production and survival of native fish.

DEQ has identified the Pahsimeroi River as not supporting the beneficial uses of salmonid spawning and cold water aquatic life as defined in state water quality standards and the federal Clean Water Act. Additionally, the Pahsimeroi River is an important component of the upper Salmon River Bull Trout recovery unit for the state of Idaho. The Endangered Species Act requires that conservation plans be developed and implemented to restore anadromous and Bull Trout populations to levels that ensure their persistence in the Pahsimeroi River subbasin.

Assessments by DEQ have identified that water quality has been limited by sediment deposition in the stream channel due to streambank erosion and riparian vegetation reduction. Previous assessments by other agencies identified similar problems and sources of pollutants associated with water quality in the Pahsimeroi River subbasin.

DEQ has developed recommendations for the reduction of streambank erosion and water temperature within the Pahsimeroi River that would ultimately result in beneficial use support through improving streambank stability and subsequently increasing riparian vegetation. This TMDL recommends no changes in the wasteload allocation for the Pahsimeroi River Fish Hatchery.

Patterson and Morse Creeks are listed for flow alteration. 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.

2001 TMDL: Stream and Pollutants for Which TMDLs Were Developed

Stream Pollutants
Pahsimeroi River Sediment, temperature

2013 Addendum

This TMDL document presents an addendum to the 2001 subbasin assessment and total maximum daily load by addressing additional assessment units (AUs) in Category 5 (i.e., impaired waters) of the 2010 Integrated Report. This document also provides information that satisfies the requirements of a 5-year review of the original TMDL.

This document addresses the 19 AUs listed in Category 5 for impaired waters in Idaho’s 2010 Integrated Report. Temperature was determined to be impairing water quality in the 2 listed AUs, and temperature load allocations are provided in this document. In addition, 3 AUs received updated TMDLs using the current DEQ method for estimating shade and an additional AU was added for a temperature TMDL that was not previously listed as being impaired. Sediment was found to be impairing beneficial uses in 3 AUs, and allocations for sediment load reductions are provided in this document. E. coli was determined to be impairing water quality in 1 AU, and a bacteria TMDL is provided for restoring beneficial uses to this AU. In total, 8 AUs received new or updated TMDLs, with one of those AUs receiving multiple TMDLs (i.e., for temperature, sediment, and bacteria).

2013 Addendum: Streams and Pollutants for Which TMDLs Were Developed

Stream Pollutants
Pahsimeroi River Sediment, temperature, bacteria
North Fork Lawson Creek Sediment
East Fork Pahsimeroi River Temperature
Short Creek Sediment

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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