When ecologist Patrick Sullivan flew into the Salmon River in Alaska to conduct a vegetation study in the summer of 2019, he was excited about paddling down the pristine Arctic river. Before he and his colleague got there, however, the pilot warned that they might not see what John McPhee had described, in his best-selling book Coming Into the Country, as the “purest water I have ever seen.”

Even then, Sullivan was not prepared for the river that eventually came into view. For as far as the eye could see, the water flowed bright orange. Research would later reveal that it was too toxic for fish, most other forms of aquatic life, and shoreline vegetation to survive.

“It was a shock,” Sullivan recalls. “The pilot told us that [the river] was clear the year before. And we have photographic evidence that shows that it was clear in 2017.”

Until relatively recently, a small scattering of rivers stained by the oxidation of iron and other heavy metals could be found throughout the circumpolar world. But over the last decade, the number of rusting or yellowing rivers has been increasing dramatically, and researchers are scrambling to determine where, when, and how the phenomenon is affecting fish and other wildlife, the drinking water that Indigenous people depend on, and whether there will be an end to its expansion throughout the Arctic. 

The pH levels in some rivers in Alaska and northern Canada have dropped to 2.3, comparable to the acidity of vinegar.

Since at least 2019, the rusting and other similar river discolorations have spread to more than 200 rivers in Alaska, a growing number of rivers in the Yukon and Northwest Territories, and as Far North as the Arctic Archipelago. The pH levels in some rivers have dropped to 2.3, comparable to the acidity of vinegar. 

No one recognized what was going on until Jonathan O’Donnell, a national parks ecologist, arrived to collect fish and biological samples from the Akillik River in Alaska’s Kobuk Valley National Park in 2018. He and his colleagues noticed then how the river had turned orange compared to the clear water they had seen the previous year. When he and Brett Poulin, an assistant professor in the department of environmental toxicology at the University of California, Davis, began sampling rivers in Alaska’s Brooks Range in 2022, they found that stream discoloration in 75 locations was associated with dramatic declines in the diversity of aquatic invertebrates and fish abundance. In a tributary of the Akillik River, for example, juvenile fish such as Dolly Varden char and slimy sculpin have disappeared. 

Those findings, they suggested in their research report, have considerable implications for drinking water supplies and subsistence fisheries in rural Alaska. There’s a concern, not yet documented, that this rusting — combined with the warming of waters, which can be lethal for cold-water species — may have contributed to the collapse of the region’s chinook salmon fishery, which was so severe that in 2024 Alaska fisheries managers and First Nations in Canada signed an agreement to close all fishing for salmon on the Yukon River for seven years.

It didn’t take a lot of sleuthing to figure out what was going on.

Rusting comes from iron sulfide minerals buried in shale bedrock in permafrost. These minerals were formed when dissolved iron reacted with hydrogen sulfide gas released by volcanoes in the Arctic’s ancient past. Like carbon trapped in permafrost, minerals and metals in permafrost remain inert so long as they are not exposed to weathering, whether from water, oxygen, or bacterial activity. 

Once the permafrost thaws, though, oxygen seeps in and groundwater begins flowing in different directions. Dormant bacteria wake up and begin to oxidize the sulfide minerals, transforming them into dissolved iron and sulfate, which in turn produces sulfuric acid. The acidified water often takes on the orange color that leaches out of iron.

In Alaska, the most recent satellite imagery indicates that the affected rivers now cover an area more than 600 miles wide.

In Alaska, the most recent satellite imagery indicates that the affected rivers now cover an area more than 600 miles wide, according to Abagael Pruitt, a researcher in the department of environmental toxicology at University of California, Davis who is, with O’Donnell and the National Park Service, studying the impacts of thawing permafrost. 

Canadian scientists have detected rusting in the Peel and Mackenzie River watersheds in the Yukon and in the Northwest Territories. Yukon Geological Survey geologist Benoit Turcotte came upon this phenomenon serendipitously while conducting erosion studies along the Ogilvie and East Blackstone rivers in the Peel River watershed in recent years. The Peel flows into the Mackenzie, Canada’s longest river.

“Several other creeks in the region have recently turned milky, with orange deposits on the channel bed that appear to significantly modify the aquatic habitat,” he said. “This slime contained air bubbles that resemble hundreds of aquatic eyes staring at us as we walked along. I’m not a biologist, but it was hard to imagine anything living in those waters.”

The changes are an unwelcome development for Indigenous leaders and conservationists who successfully spent two decades preventing mining in the Peel River watershed, whose many rivers are among the most pristine in the world.

The rusting in northern Canada is spreading so fast that scientists at the University of Alberta and at Carleton University, in Ontario, are scrambling to get research papers published and to consult with Indigenous communities before they speak publicly about their findings. 

Geological Survey of Canada scientists Stephen Grasby, Rod Smith, and their colleagues recently published a paper describing what they have seen in groundwater and surface water draining out of the Smoking Hills east of the Mackenzie River, where shales rich in organic material have been spontaneously combusting for several thousand years. The heat from the smoldering rock has long contributed to local ground slumping and permafrost thaw — a natural process — but warming-related wildfires and heavy rainfall in the region now appear to be sending this process into overdrive. 

“It’s humbling to see how nature is producing some of the most toxic acidic waters you can find anywhere,” says a scientist.

Six years after he visited the site in 2017, Grasby says he was struck by how quickly the landscape had changed. He and his colleagues found that water draining out of some places was too acidic for their pH meters to measure. The optimal pH range of drinking water is between 6.5 and 8.5. His samples registered in the negative range. “It’s humbling to see how nature is producing some of the most toxic acidic waters you can find anywhere — far worse than humans can ever make at a mining site,” he says.

Grasby says that the acidification of the water, whether it is linked with combustion or a warming climate, is also releasing toxic heavy metals such as cadmium. In one case, his team found cadmium levels in the water around the Smoking Hills to be 7,000 times higher than safe drinking water standards. 

It’s a serious concern because many studies have already detected a surge in heavy metals — including cadmium, mercury, and selenium — in salmon, seabirds, caribou, beluga whales, polar bears, and other animals. Many scientists assumed that much of this was coming from receding glaciers and ice sheets in regions such as the southwest coast of Greenland and Svalbard. Only recently has it been shown that thawing permafrost is transporting heavy metals into rivers and lakes. In a recent study, Canadian scientist Elliot Skierszkan and his colleagues suggested this kind of thawing “underscores the urgent need” for interdisciplinary research to better understand this emerging issue.

The rivers’ acidity and toxicity levels lessen as water travels downstream and gets diluted by other, cleaner sources, Grasby says. But even then, the researchers found that the drinking water they were drawing from the nearby Horton River, which receives water that drains from the Smoking Hills, exceeded levels considered to be safe. Fortunately, there are no communities situated along the Horton. But the river is a popular route for wilderness canoers.

The ground slumping that Grasby and Smith are seeing is increasing exponentially throughout the Arctic, experts say, and it’s bound to exacerbate the rusting of rivers that have similar geological features. 

Queen’s University scientists Melissa Lafrenière and Scott Lamoureux have been documenting a similar scenario in two lakes at the Cape Bounty Arctic Watershed Observatory research site on Nunavut’s Melville Island, in the highest reaches of the Arctic. 

“What we have seen is a notable change in sulfate in both East and West lakes,” says Lafrenière, who is working with a graduate student to identify the source of the sulfate. “My hypothesis is [it] comes from the oxidation of sulfide minerals that are being made accessible due to oxidation and subsurface flow of groundwater that comes with deep thaw during extremely warm years. It’s similar to what is causing the rusting rivers in the Yukon and Alaska.”  

“This is going to have a big impact on fish and aquatic life [and] the bears and raptors that feed on the fish.”

Other than protecting regions that are most vulnerable to permafrost thaw from mining and other forms of resource development that disturb the land, scientists say there’s not much that can be done about the rusting. 

“It may eventually run its course once the unweathered sulfite minerals have been oxidized,” said Sullivan. “We don’t know. But until then, this is going to have a big impact on fish and aquatic life, the bears and raptors that feed on the fish, the vegetation that grows along these rivers, and potentially the safety of water that people living downstream drink.”