Alaska’s Brooks Range: Over 75 Rivers Have Turned Orange and Some Fish Have Disappeared

To the north of Alaska, streams that had long remained clear take on a rusty hue as the permafrost thaws. Two studies published in 2024 and 2026 now describe the chemical scale of the phenomenon and its effects on wildlife. A record of pollution without factories or mines, revealing an Arctic in full transition.

In 2024, a team from the National Park Service documented more than 75 rust-coloured waterways across about 1,000 kilometers of the Brooks Range

Jonathan O’Donnell, an ecologist with the National Park Service, initially reached out to bush pilots, guides and Indigenous communities. In this way, his team compiled more than 75 reports of orange streams spread across roughly 1,000 kilometers of remote terrain. These efforts, conducted with the USGS and the University of California, Davis, were published in May 2024 in Communications Earth & Environment.

Next, satellite imagery captured between 1985 and 2022 was used to date the changes. Using a reddening index, they place the coloration in the last decade, a period of rapid warming. Indeed, the Arctic warms nearly four times the global average.

Thawing permafrost exposes sulfides that oxidize, acidify the water and release iron, zinc, nickel and cadmium

The mechanism starts with sulfur-bearing minerals, such as the pyrite, long sealed in frozen soil. With thaw, these sulfides meet oxygen and water. Their alteration then produces acidity and sulfate, which mobilize metals toward the aquifers and then the streams.

Compared with reference clear-water streams, the orange ones show a lower pH, cloudier water, and higher levels of iron, nickel, zinc, cadmium and copper. The oxidized iron explains the color. However, the 2.3 pH sometimes cited corresponds to the most degraded samples, while these rivers typically display a pH close to 8.

In the Akillik Basin, a stream tracked since 2017 lost its Dolly Varden and slimy sculpins after turning color in 2018

The most telling case concerns a small tributary of the Akillik Basin, in Kobuk Valley National Park, monitored since 2017. In 2018, it had changed color, providing scientists with a rare before-and-after comparison.

According to USGS and the National Park Service, the juvenile Dolly Varden and the slimy sculpins disappeared from this stretch. Moreover, the diversity of macro-invertebrates, the base of the food web, fell markedly. More broadly, the study links the coloration to a clear decline in fish abundance.

These losses worry communities, because the Dolly Varden, sockeye salmon and whitefish feed the villages of northwest Alaska. Additionally, metals carried downstream threaten some drinking water reserves.

Published in September 2026 in AGU Advances, a new study shows that pollution persists far downstream from the sources

The team led by Taylor Evinger, which includes Jonathan O’Donnell and the USGS, analyzed long-term measurements. According to the synthesis published by the American Geophysical Union, sulfate and zinc surged after an unusually mild winter 2019. Moreover, manganese, nickel, cadmium and rare earth elements rise downstream of seeps.

On the Salmon River, certain metals may have reached as much as 70 times their usual levels, well away from the sources. The phenomenon thus extends beyond the headwaters. An accompanying article notes similar trends in the Canadian Yukon, on sedimentary rocks rich in sulfides.

Thus, these rust-coloured streams stand as sentinels, proof that a supposedly inert frozen ground can become a source of contamination.