Gallatin River Goes Neon-Green with Algal Blooms for Fifth Consecutive Year

Montana Dept. of Environmental Quality Needs to Identify Nutrient Pollution as the Causal Agent for Algal Blooms; Public Encouraged to Submit Comment to Protect River

Neon-green algal blooms are plaguing the world-class Gallatin River downstream of Big Sky and the Yellowstone Club for the fifth consecutive summer. 

Excessive nutrient loading from development and inadequately treated wastewater from the Big Sky area, combined with long summer days and hot temperatures, has again caused a widespread algal bloom, degrading water quality and aquatic habitat in the mainstem Gallatin. 

“Five years of algal blooms on the Gallatin River are a clear sign that we need a durable solution for continued nutrient pollution problems plaguing our prized river. Thankfully, we have the tools to prevent future blooms and restore the Gallatin’s waterway health,” said Guy Alsentzer, Executive Director of Upper Missouri Waterkeeper. “Now is the time for anyone who values a clean and healthy Gallatin River to tell the Montana Department of Environmental Quality that they support a proposed river impairment listing which clearly identifies nutrient pollution as the key pollutant causing algal blooms and seasonally degrading the world-class Gallatin River.”

View Upper Missouri Waterkeeper’s photos and video here documenting another neon-green algal bloom on the Gallatin River this August, 2022. Media interested in additional photos and drone video footage can email

The DEQ has an open public comment period until August 22 on the preliminary impairment designation for the Gallatin River. The public and businesses can and should speak up for the health of this world-class river by submitting comments in support of a science-based clean-up plan that identifies the causal nutrient pollution impairing the Gallatin.


Recurrent algal blooms on the mainstem Gallatin below the resort community of Big Sky, Montana are historically rare. However, since 2018 algal blooms have become the rule, not the exception.  

Widespread algal blooms on the Gallatin River are a serious concern because they indicate the river has reached an ecological tipping point where there is no more dilution capacity, and negative ecological impacts are beginning to occur. Nutrient pollution is directly linked to causing and contributing to summer algal blooms in Montana rivers. Algal blooms degrade protected uses of Montana waterways, from recreation (fishing, swimming and boating) to aquatic life (maintaining good water quality and high quality habitat for sensitive salmonid species and macroinvertebrate communities).

In June 2022, the Department of Environmental Quality (DEQ) made a preliminary finding that the Gallatin River merits a Category 5 impairment designation under Section 303(d) of the federal Clean Water Act (CWA). The determination came in response to a petition submitted by Upper Missouri Waterkeeper, Gallatin River Task Force, Montana Trout Unlimited, Greater Yellowstone Coalition, and American Rivers late March requesting the middle segment of the Gallatin River be deemed ‘impaired’ by nutrient pollution.

In its decision, DEQ found that the Gallatin River is substantially affected by human-made nutrient pollution and its designated uses (such as recreation and aquatic life) have not been attainable due to water quality degradation caused by excessive levels of nitrogen and phosphorus. The agency, however, failed to identify nutrients as the causal agents for algal bloom impairment on the Gallatin, despite the fact that algal blooms do not occur if it weren’t for unnatural contributions of nutrients to the river.

Upper Missouri Waterkeeper and the Montana Environmental Information Center are also litigating the Montana DEQ’s 2021 approval of yet more sprawl development and new nutrient wastewater loading to the Gallatin River from the Lazy J South subdivision.  The Lazy J South subdivision was approved by the DEQ to discharge up to 20,000 gallons of poorly treated wastewater per day into groundwater in Big Sky’s Canyon Area. Best available science indicates that groundwater underlying the Canyon is hydrologically-connected to and discharges into the mainstem Gallatin River. Thus, adding more nutrient pollution from more sprawl development into local groundwater threatens to worsen the existing problem of nutrient-caused algal blooms in the Gallatin River. Waterkeeper remains committed to ensuring that DEQ sees the forest for the trees and recognizes – and acts upon – best available science showing all man-made sources of nutrient pollution in Big Sky can negatively affect river health.

The proposed impairment determination would be a win-win outcome for the Gallatin, local communities, businesses, and all who care for this world-class waterway.  A formal impairment designation for nutrient-caused algal blooms on the mainstem Gallatin would kick off a non-partisan, science-based investigation of all sources of nutrient pollution causing or contributing to recurrent summer algal blooms in the Gallatin River and inform the creation of mandatory pollution control and clean-up plans, likely in the form of nutrient TMDLs.