Once crystal clear, rivers in Alaska’s remote Brooks Range are turning orange and cloudy with dangerous levels of metals, threatening fish and fragile Arctic ecosystems, according to a new study.
The research, published in Proceedings of the National Academy of Sciences, reveals that warming temperatures are thawing permafrost – soil that has remained frozen for millennia. The thaw exposes sulphide-rich rocks to water and oxygen, producing sulphuric acid that dissolves metals such as iron, cadmium and aluminium into rivers. And it's this that changes the colour of the water, to the point where researchers say some watercourses are so orange they're visible from space.
Scientists say this geochemical shift is happening across multiple Arctic watersheds – including in Alaska’s Salmon River in the far north of the US state – putting fish, wildlife and Indigenous communities at risk.
“I have worked and travelled in the Brooks Range since 1976, and the recent changes in landforms and water chemistry are truly astounding,” says David Cooper, a research scientist at Colorado State University and co-author of the study.
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Alaska's toxic rivers
The discovery was first made in 2019 when University of Alaska ecologist Paddy Sullivan noticed unusual river conditions while studying Arctic forest shifts.
A pilot flying Sullivan into the field warned him the Salmon River looked “like sewage” and hadn’t cleared after snowmelt. Alarmed by what he found, Sullivan and a team of researchers decided they wanted to find out exactly what was going on.
Their research confirmed that thawing permafrost is fuelling reactions that oxidise rocks such as pyrite, releasing toxic metals that are harmful to many animals.
Cadmium, for example, can build up in fish organs and potentially harm predators such as bears and birds. Though current metal levels in edible fish tissue are not considered hazardous to humans, river health is already deteriorating.
Iron-rich sediment also clouds the water, blocking sunlight and smothering insect larvae essential to salmon and other fish species. Salmon, a key subsistence resource for Indigenous communities, may struggle to spawn in gravel beds clogged with fine sediment, while other fish like Arctic grayling and Dolly Varden trout are also at risk.
“It’s not just a Salmon River story,” says Tim Lyons, a biogeochemist at the University of California who worked alongside Sullivan. “This is happening across the Arctic. Wherever you have the right kind of rock and thawing permafrost, this process can start.”
Lyons says what's happening is similar to acid mine drainage – the flow of acidic water from metal and coal mines. “But here, there’s no mine,” he says. “The permafrost is thawing and changing the chemistry of the landscape.”
Unlike pollution from mines, which can be managed with treatment systems, the widespread nature of these natural chemical reactions makes mitigation nearly impossible, says Lyons: “There’s no fixing this once it starts. It’s another irreversible shift driven by a warming planet.
“There are few places left on Earth as untouched as these rivers. But even here, far from cities and highways, the fingerprint of global warming is unmistakable. No place is spared.”
Top image: the Salmon River in Alaska now runs a rusty orange thanks to metal contaminants unleashed by thawing permafrost. Credit: Taylor Rhoades
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