Study shows impact of pollution on water quality of Clark Fork River

HELENA – Recent results from an ongoing water quality study on the Clark Fork River show that pollution is more prevalent than initially thought.

Preliminary results of the new water studies show the presence of toxic compounds, such as polychlorinated biphenyls (PCBs), dioxins and furans, which are compounds associated with industrial activities, including the paper industry, but are also from some natural sources like forest fires.

In 2023 biologists with Montana Fish, Wildlife & Parks sampled for toxins on the Clark Fork River from Silver Bow Creek, near Butte, to Cabinet Gorge Reservoir on the Idaho border, including samples from the Clark Fork’s major tributaries – the Flathead, Bitterroot and Blackfoot rivers.

The compounds discovered are more widespread than previously thought. This includes their presence in headwater areas and also downstream of the Flathead River, which was previously the lower boundary of a fish consumption advisory issued in 2020.

FWP staff collected both water and fish samples looking for pollutants.

Depending on the results of the fish samples, FWP will work with the Montana Department of Environmental Quality and the Montana Department of Public Health and Human Services to evaluate a further need for an advisory on fish consumption. Currently, there is a fish consumption advisory on a 148-mile stretch of the mainstem of the Clark Fork River from the Bitterroot to the Clark Fork’s confluence with the Flathead River.

“Dioxins, furans and PCBs are stored in the fat and muscle of fish” said FWP fisheries biologist David Schmetterling. “Although they do pose risks for developmental problems, reproductive issues, and even endocrine system problems in fish, the main issue is with human consumption of the fish.”

To date, the recent findings have not been correlated with any particular site or source of contamination, and continued monitoring and investigation will help identify potential sources of the pollution. The current studies follow up on earlier studies of toxic compounds in the river system. For more than a century, water quality concerns in the Clark Fork River focused on the effects of heavy metals from hard-rock mining. After Smurfit-Stone closed its paper mill near Frenchtown in 2010, FWP biologists conducted preliminary surveys to determine if any contaminants associated with the pulping and paper industry were found in fish tissue downstream of the mill. This led to fish consumption advisories for northern pike and rainbow trout.

In 2018 and 2019, FWP worked with the Environmental Protection Agency (EPA) to collect fish tissue samples from a larger area, and results led to a wider fish-consumption advisory.

The current studies are funded through a grant from the EPA. FWP worked with several partners to secure the grant of more than $221,000. Partners include the Confederated Salish and Kootenai Tribes, Trout Unlimited, Missoula County Health Department, the Clark Fork Coalition, and the Montana Department of Justice Natural Resource Damage Program.

For more information, contact FWP Fisheries Pollution Biologist Trevor Selch at 406-444-5686 or