Protecting Public Health and the Environment.

Salmon Falls Creek Subbasin

Subbasin at a Glance

Hydrologic Unit Code 17040213
Size 863 square miles (552,320 acres) in Idaho
2,082 square miles (1,332,480 acres) total

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

Big Creek, Browns Creek, Cedar Creek, Cedar Creek Reservoir and tributaries, China Creek, Corral Creek, Cottonwood Creek, Devil Creek, Hot Creek, House Creek, North Fork Salmon Falls Creek, Player Creek, Salmon Falls Creek, Salmon Falls Reservoir, Shoshone Creek

Beneficial Uses Affected Cold water aquatic life, salmonid spawning, secondary contact recreation
Major Land Uses Rangeland, irrigated agriculture
Date Mid-Snake River TMDL Approved by EPA April 1997
EPA Approval Letter
Date Modification Approved by EPA September 2005
Date Approved by EPA February 2008

Subbasin Characteristics

The Salmon Falls Creek subbasin is located in south-central Idaho. Water quality in the subbasin, in general, is of good to moderate quality. However, Salmon Falls Creek Reservoir is one of the few water bodies in the state with identified mercury contamination. Sediment, nutrients, and temperature are the most common listed pollutants throughout the subbasin.

1997 Middle Snake River Watershed Assessment and TMDL

This watershed spans two subbasins, as summarized in the table below.

Watershed at a Glance

Hydrologic Unit Codes

17040212 (Upper Snake-Rock Subbasin)
17040213 (Salmon Falls Subbasin)

Size 94 square miles (60,160 acres)
Beneficial Uses Affected Aquatic life, primary and secondary contact recreation
Major Land Uses Irrigated agriculture, confined animal feeding operations, food processing, aquaculture, urban, hydroelectric development

A large portion of the economy and culture of south-central Idaho is dependent on water provided by the middle Snake River and its tributaries. The middle Snake River has 14 segments listed as priority segments on the 1996 §303(d) list.

The middle Snake River's hydrologic system is shaped by precipitation, the river itself, tributaries, irrigation return flows, ground water flow, and geothermal sites. With the exception of precipitation, all of these sources receive nutrient inputs from human activities. Severely diminished instream flows have historically limited the middle Snake River's ability to assimilate these nutrient-rich inputs.

This document is the first phase in a phased TMDL and focuses on reductions in total phosphorus. Proposed industry total phosphorus reductions will be implemented within 5 years of the approval of this TMDL and will be maintained for an additional 5 years to reach an instream target of 0.75 milligrams per liter total phosphorus at Gridley Bridge in Hagerman, Idaho. Total phosphorus reductions will come from aquaculture, food processors, municipalities, confined animal feeding operations, irrigated agriculture, and the hydroelectric industry.

Additional phases of the phased TMDL focus on sediment reduction (phase II), nitrogen reduction (phase III), flow (phase IV), and other pollutants and stressors (phase V). These phases have been addressed simultaneously in the Upper Snake Rock TMDL.

1997 TMDL: Streams and Pollutant for Which TMDLs Were Developed

14 sections of the middle Snake River, including Bliss, Shoshone Falls, Upper Salmon Falls, and Lower Salmon Falls Reservoirs
Total phosphorus

Aquaculture Wasteload Allocations

Draft wasteload allocations for aquaculture facilities were developed in July 2004. These allocations are designed to meet the total phosphorus reductions as specified in the Middle Snake River and Upper Snake Rock TMDLs. The allocations affect 37 TMDLs for total phosphorus and total suspended solids and six associated segments of the Snake River.

Public comments were accepted on the document in August 2004. Based on the information and comments received, DEQ modified the document and resubmitted it for public comment in the following three parts:

2007 Subbasin Assessment and TMDL

In most of the listed assessment units, total phosphorus was a limiting nutrient. However, in the Big Creek and Cottonwood Creek watersheds, it was determined that total phosphorus was in excess and may be impacting the beneficial uses of Shoshone Creek.

While the majority of stream systems appear to have little impact from excess nutrients, the same cannot be said for the two reservoir systems. Both Cedar Creek and Salmon Falls Creek Reservoirs contain excess nutrients that lead to nuisance aquatic vegetation blooms. TMDLs were developed for the tributary systems that feed these two reservoirs. Only nonpoint sources and natural soil-associated phosphorus contribute to this increase in total phosphorus concentrations, as there are no point sources in the watershed.

In the lower section of Salmon Falls Creek, where ground water plays a significant role in the hydrology of the system, nitrogen was in excess and could lead to nuisance aquatic vegetation growths.

It is EPA policy that flow and habitat alterations are pollution and not specific pollutants, and TMDLs are not required. However, streams found to be impacted by these forms of pollution will remain on the §303(d) list. Cedar Creek below the Cedar Creek Reservoir falls into this category for flow alteration.

All streams listed for temperature were examined and require load reductions as a result of lack of shade.

Although not currently listed on the §303(d) list, Salmon Falls Creek Reservoir was examined due to a fish consumption advisory placed on the water body in 2001. Fish tissues were collected in October 2006. Mercury concentrations found in fish at that time averaged well above DEQ's fish tissue criterion. In order to achieve the water quality standard, mercury levels need to be reduced by nearly 70%.

In addition, DEQ proposed removing the following water bodies from the state's list of degraded waters for specific pollutants for which TMDLs have already been completed:

  • Lower Salmon Falls Creek: bacteria and dissolved oxygen
  • Shoshone Creek: bacteria
  • Hot Creek: sediment
  • Cottonwood Creek: dissolved oxygen

2007 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Lower Cedar Creek
Temperature, sediment
Lower Salmon Falls Creek
Temperature, nutrients, sediment
Devil Creek
Cedar Creek Reservoir
Temperature, nutrients, sediment
China Creek, Corral Creek, Whiskey Slough
Temperature, nutrients, sediment
Salmon Falls Creek Reservoir
Mercury, nutrients
China Creek
Temperature, nutrients, sediment
Salmon Falls Creek
Temperature, nutrients, sediment
North Fork Salmon Falls Creek
Shoshone Creek
Temperature, sediment
Hot Creek
Big Creek
Temperature, nutrients, sediment
Cottonwood Creek
Temperature, nutrients, sediment, bacteria

Subbasin Documents

Staff Contacts

Surface Water Quality Manager
Sean Woodhead
DEQ Twin Falls Regional Office
650 Addison Avenue West, Suite 110
Twin Falls, ID 83301
(208) 736-2190

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