Why Arctic streams are turning orange

Something is happening to the streams of the Brooks Range in Arctic Alaska. They are turning bright orange, the fish are going, and the villages downstream drink from the rivers those streams feed.

An orange tributary of the Kugororuk River. The colour is iron, and it usually travels with toxic concentrations of heavy metals. USGS / Josh Koch.

Nobody was looking for it

"We first noticed an orange stream in 2018 during regular Arctic stream monitoring," says Jon O'Donnell, an ecologist with the Park Service's Arctic Inventory & Monitoring Network and lead author of the study. "We thought it was anomalous."

It was not. To find out how far it went, O'Donnell and colleagues from the Park Service, the USGS and several universities asked the people who fly over and walk through that country — bush pilots, wilderness guides, other scientists, and rural and Indigenous communities. That produced more than 75 orange streams across more than 600 miles of the Brooks Range.

Dating it from orbit

Knowing where was not knowing when. So they pulled Landsat imagery of the region from 1985 to 2022, taking July and August scenes with less than 30% cloud, dropping anything with wildfire smoke in it, and scoring each one on a Redness Index. Above 1.5, the water has gone from clear to orange.

Orange water mixing with clear. NPS / Ken Hill.

The answer is the last decade — the same decade in which Arctic air and ground temperatures jumped past the 30-year record and past the freezing threshold for near-surface permafrost. Most of these streams were clear until recently.

What is actually in the water

Between June and September 2022 the team measured metal concentrations, pH and temperature in orange streams and in clear ones nearby, and compared invertebrates and fish before and after the change using their own monitoring records.

They found iron, nickel, zinc, cadmium and copper. The orange is oxidised iron. Cadmium, nickel and manganese exceeded EPA or WHO recommendations; arsenic and lead did not.

How thawing ground turns a stream orange, and what it does downstream. Illustration by Julia Ditto, from O'Donnell et al., 2024.

The mechanism is ordinary chemistry in an extraordinary place. Thawing permafrost lets water reach deeper, exposing sulfide minerals such as pyrite to weathering for the first time in thousands of years. That releases acid, iron, sulfate and trace metals into the groundwater and then into the streams.

"This kind of acid rock drainage is what you expect to see at a mining site," says Brett Poulin of UC Davis, a co-author, "not in the remote and pristine Brooks Range in Alaska."

One stream, measured through the change

The Akillik River in Kobuk Valley National Park had been monitored since 2017. In 2018 it turned orange. The pH dropped, the metals rose, and the stream lost two fish species outright — juvenile Dolly Varden and slimy sculpin. Aquatic invertebrates declined with them.

"In a way, we lucked out," O'Donnell says, "by having biological data before and after that stream changed color, so we were able to directly assess impact to aquatic life."

Why it matters past the science

Arctic rivers carry the fish that subsistence harvesters, anglers and commercial fisheries depend on, and those fish are already under pressure from a warming climate. Metals released by thawing ground are one more. Dolly Varden, chum salmon and whitefish lose habitat; the metals carry downstream into the rivers villages take their drinking water from.

This is not a spill with a responsible party. It is leaching, it is widespread, and nobody yet knows how long a stream stays orange once it turns. "We are still learning about what makes certain rivers vulnerable to rusting," O'Donnell says. "The underlying causes are complex."

Sources

O'Donnell, J., et al. (2024), in
Communications Earth & Environment.
Written from Nina Chambers' account in Park Science 38(1), published by the
National Park Service and, as a work of the United States government, in the
public domain. Research funded by the USGS–NPS Water Quality Partnership
Program and the NPS Inventory & Monitoring Division.