Water quality study finds toxic compounds more widespread than previously thought in Clark Fork

Preliminary results from water quality testing done last summer on the Clark Fork River released this week show toxic compounds in the river are more widespread than state officials previously thought, which could lead to a reassessment of the fish consumption advisory that has been in place since 2020.

The passive water monitoring study was done at 19 sites in July and August from between where Silver Bow Creek enters the Clark Fork all the way down to Cabinet Gorge Reservoir on the Idaho border, including the Flathead, Bitterroot, and Blackfoot rivers.

The preliminary results show there are toxic compounds in the river – including polychlorinated biphenyls (PCBs), dioxins and furans – that are products of both industrial activities that have historically occurred along the river and natural sources, including wildfires.