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Snake (Lower)-Asotin Subbasin

Data Details
Hydrologic Unit Code 17060103
Size 180 square miles (115,382 acres)

Water Bodies with EPA-Approved TMDLs (Category 4a)

Tammany Creek
Beneficial Uses Affected Cold water aquatic life, secondary contact recreation
Major Land Uses Dryland agriculture, livestock grazing, urban and rural development
Date Tammany Creek TMDL Approved by EPA February 2002
EPA Approval Letter
Date Tammany Creek Addendum Approved by EPA

December 2010
EPA Approval Letter

Subbasin Characteristics

A small portion of the Lower Snake-Asotin subbasin falls within Idaho, just south of the town of Lewiston and at the northern edge of the Salmon Basin.

2001 Tammany Creek Watershed Assessment and TMDL

Tammany Creek is located in the Lower Snake-Asotin subbasin. The watershed is approximately 35 square miles (22,400 acres). Due to the extensive rural and agricultural development within the Tammany Creek watershed, surface and streambank erosion are the dominant sources of sediment loading to Tammany Creek.

During the TMDL investigation, nutrients and pathogens were also examined. Results for nutrient monitoring indicate that high nitrogen levels occur in Tammany Creek. High nitrogen levels were not correlated with changes in flow or seasonal variation, which indicates that nitrates are originating from a constant source, such as ground water recharge. Phosphorous levels remained relatively low for most of the year and were found to be the limiting factor to nuisance aquatic growth. Phosphorus levels did increase during high-flow events, indicating that phosphorous transport occurs in correlation with increased sediment transport in the watershed. Nutrient reductions for Tammany Creek will be addressed through this TMDL since phosphorus levels will decrease when sediment levels are reduced as part of TMDL implementation.

Initial pathogen monitoring results indicate that pathogen loading within Tammany Creek exceeds the Idaho water quality standards. Upon analysis, concentrations of pathogen loading appear to be somewhat random and unpredictable. Further monitoring will be conducted by DEQ to determine if a pathogen TMDL is warranted.

2001 TMDL: Stream and Pollutant for Which TMDLs Were Developed

Stream Pollutants
Tammany Creek Sediment

2010 Tammany Creek Addendum

This addendum analyzes current water quality data for the Tammany Creek watershed and amends the 2001 TMDL by establishing nutrient and E. coli bacteria TMDLs. Based on the available instream monitoring data generated at the monitoring station, nutrient TMDLs were developed for total phosphorus (TP) and nitrite plus nitrate as nitrogen (NO2+NO3-N). A bacteria TMDL was developed for E. coli bacteria. Additionally, the original sediment TMDL was revised to establish a wasteload allocation (WLA) for point sources of sediment to Tammany Creek.

2010 Addendum: Stream and Pollutants for Which TMDLs Were Developed

Stream Pollutants
Tammany Creek Total phosphorus, nitrite plus nitrate as nitrogen, E. coli

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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