DEQ to formally list Big Hole River as impaired for eutrophication and dissolved oxygen
HELENA—The Montana Department of Environmental Quality (DEQ) has determined aquatic life and recreational uses in the Big Hole River are affected by eutrophication, or the presence of excessive nutrients and low dissolved oxygen, and will formally list the Big Hole as impaired by these pollutants in its next Integrated Report.
The presence of excessive nutrients in a waterbody encourages plant and algae growth, often leading to negative impacts to fish and recreation. Waterbodies are more sensitive to nutrient increases when temperatures rise and water flow decreases.
The first streams in the Big Hole were listed on the state’s impaired waters list in 1988. In total, DEQ has completed 91 Total Maximum Daily Loads (TMDLs) on the Big Hole River and its tributaries for various pollutants, including nutrients. These TMDLs qualify the Big Hole for federal funding and inform watershed restoration plans. Since 1988, many water quality monitoring projects have occurred and the Big Hole watershed has received $1,350,590 in Clean Water Act funding to implement projects that address sediment, temperature, flow and habitat impairments. These projects are proven practices that address sources of pollution that often also contribute nutrients.
In February 2025, Upper Missouri Waterkeeper and Save Wild Trout, formerly the Big Hole River Foundation, petitioned DEQ to assess beneficial uses, list the river as nutrient impaired, and prioritize development of TMDLs for three segments of the mainstem Big Hole River using numeric nutrient standards. In DEQ’s response to the petition on April 14, 2025, the agency explained that it was unable to proceed with an assessment using numeric nutrient standards and requested additional information to proceed with a narrative nutrient assessment. Since that time, DEQ has been working with Save Wild Trout to submit and review the necessary data. The final data package, including aquatic insect community measurements, water chemistry, dissolved oxygen data, pictures, video and visual aquatic plant evaluation field forms, was provided to DEQ January of 2026. This information was paired with DEQ’s algae measurements for a comprehensive analysis.
“We’d like to thank Save Wild Trout for their diligence in collecting and sharing the data we needed to make this determination,” said Water Quality Planning Bureau Chief Andy Ulven. “Going forward, DEQ will continue to support, and would encourage others to support, restoration projects driven by local watershed planning by organizations such as the Big Hole Watershed Committee, Save Wild Trout, and Trout Unlimited.”
DEQ will list the impairments in the next Integrated Report which should be released for public comment in 2027. Depending on the nature of the comments received, DEQ may make changes to the report before submitting it to EPA. Once EPA reviews and approves the report the impairments become official. In the meantime, the watershed continues to be eligible for clean water act restoration funding.
DEQ is currently accepting public comment on the draft assessment method utilized for the impairment listings through September 11th, 2026. Specifically, the draft method is for eutrophication in wadeable streams and medium rivers. The assessment process checks for human-influenced sources, then evaluates algae and aquatic insects. Nutrient data are considered as supporting information. After an assessment, waterbody goals are developed based on site-specific data for pollutants that are contributing to eutrophication. To view the draft assessment method, visit DEQ’s public notice.
More information about the Big Hole River watershed, intended action, and the response to the petition and can be found on DEQ’s Water Quality Planning Projects page on DEQ’s website.
References and links in this press release may not work once public comment periods and documents are no longer active. Please submit an information request if you are looking for documents that are not available through the DEQ website.
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