Once-pristine Arctic rivers are turning orange. Scientists are worried.

"According to a Sept. 8 paper published in the proceedings of the National Academy of Sciences, an iconic Arctic watershed in Alaska's Brooks Range has recently turned a dark, murky orange color, alarming scientists throughout California and Alaska. "This is what acid mine drainage looks like," Tim Lyons, a biogeochemist at UC Riverside, told the university in a September 2025 article. "But here, there's no mine. The permafrost is thawing and changing the chemistry of the landscape." According to the research paper, authors believe that permafrost - soil, rock or sediment that's been frozen for at least two years - is thawing, exposing sulfide minerals and delivering iron and other potentially toxic metals to the remote Alaskan wilderness."

""I have worked and traveled in the Brooks Range since 1976, and the recent changes in landforms and water chemistry are truly astounding," David Cooper, Colorado State University research scientist and study co-author, told UC Riverside. According to the university, one of the authors first noticed the murky waters in 2019, when a pilot flying them to another research destination remarked that the Salmon River looked "like sewage" ever since the recent snowmelt."

In 1975 the Salmon River area featured exceptionally clear, pristine waters supporting Arctic grayling, chum salmon and Dolly Varden. By 2019 pilots and researchers began noticing the river's color shift to a dark, murky orange following spring snowmelt. A September 2025 paper links the color change to thawing permafrost that exposes sulfide minerals and mobilizes iron and other potentially toxic metals, producing water chemistry resembling acid mine drainage despite the absence of mines. Field observations from long-term regional scientists characterize recent changes in landforms and water chemistry as dramatic and alarming.

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