Remote Alaska Rivers Flow Orange With Metals Released From Thawing Permafrost; Multiple Rivers Now Toxic To Fish [View all]

The writer John McPhee once described Alaska’s Salmon River as having “the clearest, purest water” he’d ever seen. Today, that same river runs orange with toxic metals unleashed by thawing permafrost. “During the summer of 2019, the clear waters of the Salmon turned distinctly orange and have remained discolored and turbid since,” according to a new study published in the Proceedings of the National Academy of Sciences.


A tributary of the Kugororuk River runs orange in 2023. Photo by Josh Koch, U.S. Geological Survey. Public Domain

The Salmon River’s transformation represents a much larger crisis. In Alaska’s Brooks Range,75 streams have “recently turned orange and turbid,” the study found. “This is what acid mine drainage looks like,” said Tim Lyons, a biogeochemist at the University of California, Riverside, and co-author of the study. “But here, there’s no mine. The permafrost is thawing and changing the chemistry of the landscape.”

The cause lies underground in permanently frozen soil, known as permafrost. As global temperatures rise, this ancient layer is thawing. When water and oxygen reach the newly exposed soil, they trigger chemical reactions that break down sulfide-rich rocks, creating sulfuric acid that leaches naturally occurring metals like iron, cadmium and aluminum from rocks into the river, the research shows. Researchers tested 10 major tributaries of the Salmon River and found that nine had toxic concentrations of at least one metal on at least one of three sampling dates. The study shows that levels of metals in the river’s waters exceed U.S. Environmental Protection Agency (EPA) toxicity thresholds for aquatic life.

Metal levels in the Salmon River mainstem were dangerously high throughout most of its length, and exceeded EPA chronic exposure thresholds for total recoverable iron, total recoverable aluminum, and dissolved cadmium, all the way from its first major tributary to its mouth, the study found. At one location, aluminum concentrations reached nearly five times the safe limit for aquatic life. Cadmium was present almost exclusively in dissolved form — something the study notes is “rare in aquatic ecosystems and highly toxic to aquatic organisms.” Pacific salmon (Oncorhynchus spp.) and Dolly Varden trout (Salvelinus malma) are among the most sensitive fish species to cadmium exposure, according to the research.

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https://news.mongabay.com/2025/09/alaskan-rivers-turn-orange-as-permafrost-thaws-threatening-fish-and-communities/