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Clark Fork River (Lower) Subbasin

Hydrologic Unit Codes17010213
Size247 square miles (158,080 acres)
Water Bodies with EPA-Approved TMDLs (Category 4a)Cascade Creek, Clark Fork River, Dry Creek, East Fork Creek, Johnson Creek, Lightning Creek and tributaries, Mosquito Creek, Rattle Creek, Savage Creek, Twin Creek, Wellington Creek
Beneficial Uses AffectedCold water aquatic life, salmonid spawning, primary and secondary contact recreation, domestic water supply
Major Land UsesForestry, agriculture, rural residential, recreation
Date Approved by EPAOctober 2007 EPA Approval Letter
Dates Clark Fork-Pend Orielle TMDL Approved by EPAApril 2001 EPA Approval Letter
September 2000 EPA Approval Letter
Lower Clark Fork PNV TMDL Approved by EPAJuly 2022 EPA Approval Letter

Subbasin Characteristics

Primarily located in Montana, the 320-mile long Clark Fork River flows from near Butte, Montana, to Lake Pend Oreille in Idaho. The Clark Fork River drains approximately 22,000 square miles in western Montana and northern Idaho, 247 square miles of which comprise the lower Clark Fork subbasin in northern Idaho. The river drains into the 95,000-acre Lake Pend Oreille, and as the lake’s largest tributary, the Clark Fork River contributes approximately 92% of the annual inflow to the lake and most of the annual suspended sediment load.

2007 Subbasin Assessment and TMDL

This document addresses the lower-most 247 square miles of the subbasin located in northern Idaho.

The lower Clark Fork subbasin includes 180 miles of perennial streams. The river itself flows from east to west. With approximately 75% of the subbasin in public ownership, there is a diversity of recreational opportunities and wildlife. The river’s main tributary, Lightning Creek, harbors a regionally significant Bull Trout population and supports many other native fish.

Located just downstream from the Montana/Idaho border 10 miles before the river enters Lake Pend Oreille is AVISTA’s Cabinet Gorge Dam. The Cabinet Gorge Reservoir has a storage capacity of 105,000 acre-feet at full pool.

Metals and total dissolved gas pollution are the pollutants of concern in the main stem Clark Fork River. Intensive mining around the headwaters of the Clark Fork River in Montana left residues of heavy metals behind, which still pose a risk to water quality throughout the basin. Total dissolved gas supersaturation caused by the entrainment of gas in the water when spill occurs at a hydroelectric facility can remain high for significant distances downstream from the facility. Cabinet Gorge Dam has a capacity of approximately 36,000 cubic feet per second. When river flows exceed this capacity, excess flow spills. Entrained gases from these spills can remain in the water column into Lake Pend Oreille and the Pend Oreille River.

Temperature is identified as a pollutant in the lower Clark Fork River below the Idaho/Montana border. However, the lower Clark Fork River on the Montana side of the border has not been found to violate Montana water quality standards for temperature.

Sediment and temperature are the pollutants of concern in the tributaries to the lower Clark Fork River. Thick glacial outwash sediments in steep drainages combined with timber harvest and road construction have created potential sediment problems in several of the tributaries to the Clark Fork River. Temperatures exceed water quality standards for salmonid spawning throughout the subbasin. Fire and historic timber harvest have created a more open canopy and related stream warming compared to background conditions.

TMDLs were developed for each stream determined to not fully support beneficial uses in accordance with state of Idaho water quality standards. The TMDLs included in this document address instream sediment, metal, and temperature reduction goals to maintain or restore cold water aquatic life and salmonid spawning in the tributaries. The TMDLs help quantify needed improvements and target management actions to address water quality improvement measures and timelines.

2007 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Clark Fork River (main stem in Idaho)Nutrients, sediment, temperature
Cascade CreekSediment
Dry CreekSediment, temperature
Mosquito CreekNutrients, sediment, temperature
Twin CreekSediment, nutrients
East Fork CreekSediment
Johnson CreekSediment, nutrients
Lightning CreekSediment
Rattle CreekSediment
Savage CreekTemperature
Wellington CreekTemperature

Subbasin Documents

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DEQ Coeur d’Alene Regional Office
2110 Ironwood Parkway
Coeur d’Alene, ID 83814
Ph: (208) 769-1422
Fax: (208) 769-1404
Toll Free: (877) 370-0017

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