The Salmon River, once a symbol of pristine wilderness and the subject of admiration in John McPhee's classic novel, has changed dramatically. Its once crystal-clear waters, where you could see 15 feet down, turned orange in 2019 and have since remain cloudy and discolored. This sudden deterioration in water quality in the Salmon River and many other once-clear streams in the region is a direct and worrying consequence of melting permafrost, which is closely linked to climate change.

Thawing permafrost exposes sulfide minerals in the bedrock, which are then exposed to weathering. When dissolved oxygen in groundwater meets these newly exposed minerals, such as the iron sulfide pyrite (FeS2), it can lead to the production of sulfuric acid. This acid leaches iron and other potentially toxic metals from the parent material and transports them into aquatic ecosystems. The result is a process similar to acid mine drainage, turning clear, pristine streams into orange, murky, and toxic waterways.

And the Salmon River is not alone. A recent survey in the Alaskan mountains Brooks identified 75 streams, which have recently turned orange and cloudy. The main Salmon River and most of its tributaries already exceed limits for metal concentrations considered toxic to aquatic life according to the chronic (4-day) exposure thresholds set by the United States Environmental Protection Agency (EPA). Our findings indicate that habitat quality for resident and anadromous fish has significantly deteriorated in the Salmon River watershed.

Metals that consistently exceeded EPA toxicity thresholds include: total recoverable iron, total recoverable aluminum and dissolved cadmium. Aluminum, although abundant in the Earth's crust, is highly soluble at low pH and becomes toxic to fish, accumulating on the gills and interfering with their respiration. Iron forms orange, cloudy precipitates that reduce light penetration, limit primary productivity, and damage the benthic environment and fish gills. Cadmium is highly toxic to aquatic organisms, and its accumulation on the gills of fish can lead to acute calcium deficiency and death, with Pacific salmon and Arctic char being among the most sensitive species. In addition, observations showed that sites where invertebrates were found under rocks on the stream bottom had significantly lower concentrations of total recoverable iron and aluminum.

This degradation is already having devastating effects on the ecosystem. It is thought to help explain the recent decline in chum salmon returns (chum salmon), which is a key commercial and subsistence food source in an economically disadvantaged region. Chum salmon use freshwater habitats for spawning and early life stages. Spawning habitats in the Salmon River and many other streams are now seriously threatened. The 2024 commercial catch (5,392 salmon) and the permit catch (225 salmon) were lowest since 1962, when the state of Alaska began managing fisheries in Kotzebue Bay.

Our observations point to the potential broad-spectrum future degradation of watercourses with further melting of permafrost further north and elsewhere at high latitudes. If climate change promotes melting of permafrost, a greater flux of iron and other metals into aquatic ecosystems is expected. Although the effects of metal loading on aquatic ecosystems of headwaters and tributaries are strongly negative, uncertainty remains about the impacts on larger rivers and coastal marine environments.

Mitigation strategies are urgently needed and the situation calls for immediate baseline studies in threatened but not yet impacted streams. The iconic Arctic wilderness is being transformed into a toxic environment due to climate change, with serious long-term ecological and economic consequences for local communities and the fragile Arctic ecosystem. JRi

The study is published in the journal Proceedings of the National Academy of Sciences .