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Raft River Subbasin

Data Details
Hydrologic Unit Codes 17040210
Size 1,234 square miles (789,760 acres) in Idaho 1,513 square miles (968,320 acres) total
Water Bodies with EPA-Approved TMDLs (Category 4a) Cassia Creek, Lake Fork, Raft River, Sublett Creek, Sublett Reservoir
Beneficial Uses Affected Cold water aquatic life, salmonid spawning, secondary contact recreation
Major Land Uses Range, agriculture, forest
Date Approved by EPA January 2004 EPA Approval Letter
Date Temperature TMDL Addendum Approved by EPA April 2012 EPA Approval Letter

Subbasin Characteristics

The Lower Boise River subbasin is located in southwest Idaho. The lower Boise River itself is a 64-mile stretch that flows in a northwesterly direction through Ada and Canyon Counties and the cities of Boise and Caldwell. It originates at Lucky Peak Dam and flows into the Snake River near Parma. Major tributaries include Fifteen mile Creek, Mill Slough, Mason Creek, Indian Creek, Conway Gulch, and Dixie Drain.

2004 Subbasin Assessment and TMDL

The general physical and biological characteristics of the Raft River subbasin have a strong influence on the water quality of the subbasin. Land use in the subbasin is predominantly rangeland. Irrigated agriculture (cropland and pastures) also exists in the subbasin where water is either pumped from the ground or diverted from Raft River. The major population center of the subbasin is the town of Malta. The subbasin contains two different water sources. The first is runoff from the snowpack and other precipitation events in the mountainous regions that surround the subbasin to the south, east, and west. The second is the Raft River aquifer below Malta and Almo, which is part of the Eastern Snake River Plain aquifer.

2004 Subbasin Assessment and TMDL

This document addresses the six water bodies in the Raft River subbasin that have been placed on the §303(d) list of impaired water bodies.

Nutrients are impairing Sublett Reservoir. To effectively reduce the amount of excess nutrients entering the reservoir, TMDLs should be developed for Lake Fork and Sublett Creeks, the two tributaries of the reservoir. However, it was determined that total phosphorus was not impairing the beneficial uses of the creeks. Nitrogen compounds were not found to exceed US Environmental Protection Agency recommendations within the Raft River subbasin.

Temperature is a listed pollutant in Raft River, and DEQ is proceeding with a temperature TMDL. However, DEQ is participating in a regional review of temperature criteria, after which the temperature exceedance documented in Raft River will be reassessed and, if needed, temperature TMDLs will be completed on other segments or updated on the Raft River segment (see 2012 addendum below).

Raft River, Sublett Creek, Sublett Reservoir, and Cassia Creek are listed for flow alteration; Cassia Creek is also listed for habitat alteration. The US Environmental Protection Agency does not believe that flow alteration or habitat alteration are pollutants 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 or habitat alteration.

2004 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Stream Pollutants
Raft River Temperature, bacteria, sediment
Sublett Creek Nutrients
Cassia Creek Nutrients, sediment
Fall Creek Nutrients, sediment
Lake Fork Creek Nutrients
Sublett Reservoir Nutrients

2012 Addendum

This document addresses one water body in the Raft River subbasin (Cassia Creek) that has had temperature exceedances of water quality standards, either through §303(d) listing or acquired temperature data.

The Raft River from the Utah border to Malta was listed on Idaho’s 1998 §303(d) list for temperature pollution. The US Environmental Protection Agency (EPA) added streams that exceeded Idaho’s temperature criteria to Idaho’s 1998 §303(d) list of impaired waters. However, no streams in the Raft River subbasin were among those EPA additions. On Idaho’s 2002 §303(d) list, the Raft River from the Utah border to Edwards Creek and from Cottonwood Creek to Cassia Creek was listed for temperature pollution. A temperature TMDL was completed for these two sections of the Raft River in 2004. On Idaho’s 2010 §303(d) list, Cassia Creek (assessment unit ID17040210SK005_04) was listed for temperature pollution. This document addresses the temperature TMDL for Cassia Creek.

Effective shade targets were established for Cassia Creek based on the concept of maximum shading under potential natural vegetation resulting in natural background temperature levels. Shade targets were derived from effective shade curves developed for vegetation types in southern Idaho. Existing shade was determined from aerial photo interpretation then partially field verified with Solar Pathfinder data. Cassia Creek lacks shade to some degree, although the stream is in relatively good condition. The lack of shade is likely the result of a combination of factors, including natural and human-influenced dewatering of the stream channel and historic removal of riparian vegetation associated with livestock grazing and agricultural practices. While not much can be done to address channel dewatering, most streams would recover riparian vegetation if temporarily or permanently excluded from use.

2012 Addendum: Stream and Pollutant for Which TMDLs Were Developed

Stream Pollutants
Cassia Creek CreekTemperature

Subbasin Document(s)

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DEQ Twin Falls Regional Office
650 Addison Avenue West, Suite 110
Twin Falls, ID 83301
Ph: (208) 736-2190
Fax: (208) 736-2194
Toll Free: (800) 270-1663

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