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Otter Creek TMDL Project

Page history last edited by Christina Staten 4 years, 7 months ago Saved with comment

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Project Purpose


Why DEQ Writes TMDLs

The state of Montana monitors its waters and conducts water quality assessments to determine if waterbodies are supporting their designated uses (such as aquatic life; drinking water; and agricultural, industrial, and recreational uses). Each waterbody has a set of designated uses, and Montana has established water quality standards to protect those uses. Waters that are determined not to be supporting one or more of their designated uses are called impaired and are placed on Montana’s list of impaired waters. Impaired waterbodies and their associated probable causes and sources of impairment are published within Montana’s biennial water quality integrated report.


Montana’s state law, and the federal Clean Water Act established by Congress in 1972, requires development of total maximum daily loads (TMDLs) for all waterbodies impaired by a pollutant (e.g., metals, nutrients, salinity, sediment, temperature). A TMDL is the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards (think of a TMDL as a loading rate).


TMDL Development Steps & TMDL Document Contents


TMDL development includes four main steps:

  • Characterizing the impaired waterbody’s existing water quality conditions and comparing those conditions to Montana’s water quality standards. During this step, measurable target values are set to help evaluate the stream’s condition in relation to the applicable water quality standards.
  • Quantifying the magnitude of the pollutant contribution from its significant sources
  • Determining the total allowable load of the pollutant to the waterbody (the TMDL)
  • Allocating the total allowable pollutant load into individual loads for each significant source (referred to as load allocations for nonpoint sources and wasteload allocations for point sources)


The TMDL planning process for this project incorporates a combination of water quality sampling and hydrologic modeling to further identify and quantify metals, salinity, and sediment contributions from all significant sources to Otter Creek. The TMDL document(s) will contain information from each of the above four steps, and will also contain strategies and recommended land management practices for improving water quality. Additional detail about the TMDL development process can be found on the What is a TMDL page on this site. You can also download our pamphlet: Understanding the TMDL Process (580 kb).


Project Area Location

The Otter Creek TMDL project only includes the Otter Creek watershed, which is part of the Tongue River watershed, and the greater Yellowstone River watershed. Otter Creek is located in southeastern Montana within Powder River, Rosebud, and Big Horn counties (Map 1). The Otter Creek watershed is approximately 454,000 acres (709 square miles or about 1,833 square kilometers) and is located within Lower Tongue Hydrologic Unit Code # 10090102. Otter Creek originates near the Montana – Wyoming border and flows approximately 103 miles to its confluence with the Tongue River at Ashland, MT.

Map 2 shows the extent of the project area.


Map 1: Location of the Otter Creek Watershed

(Click on map to enlarge)

Map 2: Otter Creek TMDL Project Area

(Click on map to enlarge)

Otter Creek Watershed Location Map   



Otter Creek Water Quality Impairments

Otter Creek must be maintained suitable to support non-salmonid fishes (warm water fishes) and associated aquatic life, recreational uses, and to marginally support drinking water uses, as well as agricultural and industrial water supply uses. Otter Creek was determined to be impaired in 1996 and was placed on Montana’s list of impaired waters for the pollutant causes of iron, salinity, and sediment, each of which requires a TMDL. Additional information about these impairment causes can be found on DEQ's Clean Water Act Information Center. Click on "Search" and then search by the assessment unit ID (AUID) MT42C002_020 or the waterbody name "Otter Creek" in the Middle Yellowstone Watershed.



Two rounds of iron sampling took place in 2013. The first occurred at the beginning of June, and the second at the beginning of August. A table and map of sampling locations can be found in the sampling and analysis plan (1.17 MB). The sampling results are available on the Otter Creek Documents page.


A significant amount of iron data exists from a U.S. Geological Survey sampling site near the mouth of Otter Creek, and iron data was collected in the lower portion of the watershed for the purpose of a baseline characterization for the proposed Otter Creek, LLC coal mine. DEQ conducted additional iron sampling in 2013 to gather data in other areas of the watershed for a more comprehensive look at iron sources and loading throughout the watershed. 


 An iron TMDL has been developed for Otter Creek and the "Otter Creek Iron Total Maximum Daily Load - Draft" document was available for public comment from October 1, 2015 through October 30, 2015. DEQ is in the process of responding to public comments and making related document edits. Once this public comment phase is complete, the document will be submitted to the U.S. EPA for approval. Once approved, the document will be considered final and will be posted on DEQ's Final TMDL Documents webpage. 



DEQ has completed an evaluation of existing conditions of sediment in Otter Creek, and based on a weight-of-evidence approach examining suspended sediment concentrations and discharge (flow) relationships, DEQ determined Otter Creek is not impaired for sediment and removed the sediment impairment from the 303(d) list (impaired waters list). This impairment removal is captured in Montana's 2014 Water Quality Integrated Report. A "Sediment Beneficial Use Support Assessment for Otter Creek" that examines and details this approach was published by DEQ in November 2013 and is available on the Otter Creek Documents page.


Numerous reports, sediment data, and sediment-related data were used for this evaluation, including information from: the U.S. Department of Agriculture, the Natural Resources Conservation Service, the U.S. Geological Survey, the EPA, and the Montana Natural Heritage Program. All of the information is referenced in the Beneficial Use Suport Assessment for Otter Creek, and are available on the Otter Creek Documents page. Data includes suspended solids concentration water quality samples, an analysis of erosion from upland sources and streambanks, assessments of riparian vegetation conditions, and aquatic life data. Because a significant amount of sediment data exists for the Otter Creek watershed, Because of the significant amount of existing data, DEQ did not collect additional data. 



Salinity is a measure of how much dissolved salt is in the water, and it is easily measured as conductance (electrical conductivity (EC) or specific conductance (SC)). Sodium adsorption ratio (SAR) is a measure of the suitability of water for irrigation, and is calculated using a ratio of sodium (Na) to calcium (Ca) and magnesium (Mg). As part of this project, a LSPC water quality model was developed to determine the existing load of electrical conductivity and sodium adsorption ratio in Otter Creek. The LSPC model does not specifically model salinity, but instead models cations in the water that can be used to calculate SAR. The observed relationship between the cation totals (Ca, Mg, Na) and conductance is being used to evaluate salinity. The main objective of the model is to identify relative source contributions of salinity under varying scenarios, including the condition of Otter Creek with and without current agricultural practices or urban environments (i.e., post and pre-Columbian Era).


The model's simulated results show that EC and SAR under pre-Colubian Era conditions exceed Montana's Tongue River tributary standards over 99% of the time and that current agricultural practices and other human activity have neglible impacts on EC and SAR in Otter Creek. DEQ is using the modeled results to develop EC and SAR water quality standards specific for Otter Creek. A schedule for completion of a salinity TMDL will largely be driven by development of the site-specific standards. 


For additional information about the definitions of EC and SAR, Montana’s EC and SAR water quality standards, and the application of these standards for Otter Creek, please see the Frequently Asked Questions page. Contact Myla Kelly (406-444-3639) in DEQ's Water Quality Standards Section for updates on the status of site-specific standards development for Otter Creek. 


Project Schedule

 The public comment period for the "Otter Creek Iron Total Maximum Daily Load - Draft" document closed on October 30, 2015. DEQ is in the process of preparing responses to public comments and making related document edits. Once this public comment phase is complete, the document will be submitted to the U.S. EPA for approval. Once approved, the document will be considered final and will be posted on DEQ's Final TMDL Documents webpage. 


DEQ is in the process of developing site-specific salinity standards for Otter Creek. Please contact Myla Kelly (406-444-3639), DEQ's Water Quality Standards Section Supervisor, for updates on this process. 


Project status updates and information on stakeholder and public meetings can be found on the Otter Creek Project Updates & Outreach page.



Project Contacts

A full list of project contacts, as well as contact information for the local conservation districts can be found on the Otter Creek TMDL Project Contacts page.






Project Coordinator

Christina Staten


(406) 444-2836

Project Manager: Iron TMDL

Kristy Fortman


(406) 444-7425

Project Manager: Salinity TMDL

Kristy Fortman



(406) 444-7425

Lead for site-specific salinity standards development for Otter Creek Myla Kelly MKelly2@mt.gov (406) 444-3639


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Page Released: June 5, 2013

Last Updated: October 8, 2019