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Lochsa River Subbasin

Data Details
Hydrologic Unit Codes 17060303
Size 1,180 square miles (755,200 acres)
Beneficial Uses Affected Cold water aquatic life, salmonid spawning, primary contact recreation, domestic water supply
Major Land Uses Forest
Date Approved by EPA Subbasin assessment only; not subject to EPA approval
Addendum Subbasin assessment only; not subject to EPA approval
Date Temperature TMDL Approved by EPA

August 2018
Approval Letter

August 2020 revision
Approval Letter

Subbasin Characteristics

The Lochsa River subbasin encompasses predominately undeveloped forestland and free-flowing streams. Most of the Lochsa River itself is designated as a Wild and Scenic River, and approximately half of the subbasin lies in the Selway Bitterroot Wilderness Area. The streams in the subbasin are home to a variety of fish, including Chinook Salmon, steelhead, Mountain Whitefish, and Bull Trout.

1999 Subbasin Assessment

Twenty-five tributary stream segments were §303(d) listed for sediment. Available data show that the sediment concentrations in the streams are within the normal range that is expected in the subbasin. Data also show that all 25 listed tributary stream segments support their beneficial uses. DEQ recommends all the segments be delisted for sediment.

The Lochsa River and one tributary stream segment were §303(d) listed for temperature. DEQ reviewed habitat, fish, and temperature data and concluded that, while water temperatures do exceed the state’s criteria during parts of the year, these exceedances are natural and the river and its tributaries support their designated uses. DEQ recommends that the Lochsa River and the one tributary listed for temperature be removed from the §303(d) list for temperature.

As all listed stream segments were recommended for delisting, no TMDLs were developed in this 1999 subbasin assessment.

2012 Lochsa Temperature TMDL

The 2012 Lochsa Temperature TMDL established TMDLs for 37 assessment units (AUs) within the Lochsa River watershed. Six of the 37 AUs in the TMDL were included in Category 5 of Idaho’s 2014 Integrated Report. The TMDL analysis establishes water quality targets and load capacities, estimates existing pollutant loads, and allocates load reductions needed to return listed waters to a condition meeting water quality standards.

At the request of the Idaho Department of Environmental Quality, in August of 2020, EPA amended the approval the Lochsa Temperature TMDL to include only the six assessment units that were in Category 5 of Idaho’s 2014 integrated Report. The 31 assessment units not included in the amended approval are considered informational TMDLs. Those 31 assessment units will be assessed for impairments in the future.

Subbasin Document(s)

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DEQ Lewiston Regional Office
1118 F St.
Lewiston, ID 83501
Ph: (208) 799-4370
Fax: (208) 799-3451
Toll Free: (877) 541-3304

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