New Research Shows Why Arctic Streams Are Turning Orange

In the pristine Brooks Range in Arctic Alaska, streams are turning bright orange and fish are disappearing, threatening the well-being of local communities. A recent scientific paper reveals why.

By Nina Chambers

About this article

This article was originally published in the "In Brief" section of Park Science magazine, Volume 38, Number 1, Summer 2024 (August 30, 2024).

An orange tributary of the Kugororuk River in Alaska’s Brooks Range. Scientists have found dozens of once-clear streams that have turned orange in recent years. The orange color is from iron, but it's often linked to toxic concentrations of heavy metals as well.

Image credit: U.S. Geological Survey / Josh Koch

The Arctic is warming much faster than the global average, and it’s altering terrestrial and aquatic ecosystems. Warmer temperatures are causing permafrost to thaw and release substances that have been locked in the frozen ground for thousands of years. For some compounds, such as the greenhouse gases carbon dioxide and methane, their release is largely invisible. But recent releases are very visible: streams in Arctic Alaska’s Brooks Range have turned bright orange. A recent paper in Nature Communications Earth & Environment describes how this is occurring and what it means to people who live in this remote region.

“We first noticed an orange stream in 2018 during regular Arctic stream monitoring,” said Jon O’Donnell, lead author and ecologist for the National Park Service’s Arctic Inventory & Monitoring Network. “We thought it was anomalous and set out to learn everything we could about what was happening and why.” O’Donnell and other scientists from the National Park Service, U.S. Geological Survey, and several universities have since found many more streams that have turned orange across the Arctic. As this was not an isolated event, they wanted to understand the cause of the color change, when streams started to change, and the effect on aquatic ecosystems.

Determining the Extent of the Problem

The Arctic Network consists of over 19 million acres of parklands—about a quarter of the total land area in the National Park System. Within that area, network ecologists conduct site visits to monitor a relatively small number of streams and rivers for changes in their chemistry and biology. To document the scope of the problem, O’Donnell and colleagues began by crowd-sourcing observations from bush pilots, wilderness guides, other scientists, and rural and Indigenous communities. Through this process, they compiled observations of more than 75 orange streams that span more than 600 miles (1,000 km) of remote terrain in Alaska’s Brooks Range.

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They were able to document that streams changed color in the last decade during a period of rapid warming and permafrost thaw.

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The next step was to determine when the streams turned orange, and to do that, the authors turned to satellite imagery. They collected satellite images of the area taken from 1985 to 2022, between the months of July and August, and when cloud cover was less than 30 percent. For three sites where they had conducted visits, they used the satellite images to calculate a Redness Index for each scene, removing any images that had smoke from wildfires. They identified a value of 1.5 as the threshold for when water changes from clear to orange/red.

Bird's eye view of orange stream water mixing with clear water. To pinpoint when orange streams began appearing, the study authors zoomed out even further, and analyzed satellite imagery dating back to 1985.

Image credit: NPS / Ken Hill

By looking at Redness Index values over time, they were able to document that streams changed color in the last decade during a period of rapid warming and permafrost thaw. A more general review of satellite imagery across the region supported these findings, indicating that most orange streams were clear up until the last decade. “We selected study sites where streams had recently changed from clear to orange based on both the crowd-sourced observations and satellite analyses,” said O’Donnell. “From that, we also selected sites that are relatively easy to access from hub communities like Kotzebue.” Some satellite images indicated downstream shifts from orange to clear. More research could help scientists understand why.

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“We lucked out by having biological data before and after that stream changed color, so we were able to directly assess impact to aquatic life.”

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Between June and September 2022, O’Donnell and fellow scientists measured the chemistry of both orange streams and nearby clearwater streams, including metal concentrations, pH (or degree of acidity), temperature, and other factors. They compared water chemistry, aquatic invertebrates, and fish before and after stream discoloration, based on years of monitoring data. This enabled them to determine how biological systems were being affected. “We had been monitoring a small stream in the Akillik River basin in Kobuk Valley National Park, starting in 2017,” O’Donnell recalled. “During a 2018 site visit, we noticed that this stream had changed from clear to orange. In a way, we lucked out by having biological data before and after that stream changed color, so we were able to directly assess impact to aquatic life.”

Rivers Are Rusting

O’Donnell and his colleagues found high levels of iron, nickel, zinc, cadmium, and copper in affected waters, although oxidized iron is what’s turning them orange. Movement of metals from thawed ground to water may cause a loss of habitat for important subsistence fish species like Dolly Varden, chum salmon, and whitefish. The metals transported downstream from headwater streams to larger rivers could also contaminate drinking water supplies for nearby villages.

Possible factors helping to turn Arctic streams orange and their implications for drinking water, subsistence fishing, wildlife, and ecosystems.

Image credit: O'Donnell et al., Communications Earth & Environment, 2024. Illustration by Julia Ditto.

Narrative description of "possible factors" illustration

This image is a detailed illustration of possible factors leading to the discoloration of Arctic streams, and their consequences. Several rectangular sections are overlaid on an illustration of an arctic stream flowing down from the mountains into lowlands, ending in a cross-sectional view of the stream and surrounding soils.

Upper Part of the Illustration

Common Biota of Arctic Streams (top left)

This rectangular section has three smaller sections showing different arctic stream organisms: First is algae, illustrated by a light green tuft. An arrow leads from this section into the next, which depicts macroinvertebrates: mayflies, stoneflies, midges, craneflies, and snails. An arrow leads from this section to the third section, which shows three species of fish: dolly varden (Salvelinus malma), chum salmon (Oncorhynchus keta), and Arctic grayling (Thymallus arcticus).

Upland Processes (top right)

This rectangular section depicts processes associated with the higher reaches of the stream and points to an illustration of a mountain stream.

Text reads: As permafrost thaws by active layer thickening, sulfide minerals (e.g., pyrite) are exposed to chemical weathering, releasing sulfate, acid, and trace metals into groundwater and streams.

A diagram below shows an upland stream cross-section, clear on the left, and orange on the right. There's pyrite in the permafrost on both sides, but on the orange side, more of the surface has thawed and part of the pyrite is shown leaching into the stream with flowing groundwater, blanketing the bottom with an iron (III) particulate layer. On the clear side, the groundwater is restricted to above the pyrite, and flows cleanly into the stream, which has a layer of algae present on the bottom.

Middle Part of the Illustration

Point Source

This rectangular section points to an illustration of a patch of dead vegetation in the lowland, just upslope of the stream.

Text reads: Point sources of acidic water can occur through groundwater seeps that emerge at the ground surface, likely through thermokarst processes. In some cases, this acidic water can kill tundra and boreal vegetation.

(Thermokarst is a process that occurs when ice-rich permafrost thaws, creating distinct landforms like hummocks and hollows.)

Water Quality

This rectangular section points to an illustration of a cross section of the stream.

Text reads: Orange stream reaches are more turbid, acidic, and have higher concentrations of iron particulates and other trace metals than nearby clearwater streams. Stream beds are typically blanketed with precipitated iron minerals that can impact the benthic aquatic food web.

Lower Part of the Illustration

This part depicts processes associated with the lower reaches of the stream.

Lowland Processes (bottom left)

This rectangular section points to an illustration of a cross section of lowland soils beside the stream, where groundwater is shown flowing through saturated soils above a permafrost layer, leaching orange water into the stream.

Text reads: Permafrost thaw can result in wetter soils and the release of iron from previously frozen soil. Under these conditions, iron is reduced to the highly mobile dissolved iron(II) ion, which can be transported by groundwater to streams.

Human Implications (bottom left)

This rectangular section has two numbered parts:

1. Drinking Water

Text reads: Downstream communities that rely on rivers for domestic use, including drinking water, may be impacted by iron and trace metal mobilization.An illustration to the right of the text depicts an orange water droplet falling from a faucet into a glass of water.

2. Subsistence Fishing

Text reads: Orange streams may impact subsistence fisheries through the accumulation of toxins in fish species, the loss of habitat, and degraded spawning grounds.

An illustration to the right of the text depicts a person in a small boat pulling up a fishing net with a fish in it.

Impacts to Aquatic Food Webs (bottom right)

This rectangular section points to an illustration of the lower waters of the stream and a fish. It has three numbered parts:

1. Direct uptake of trace metals by fish from the water column. 

An illustration to the right of the text depicts a cluster of orange dots with a curved arrow pointing at a drawing of a salmon.

2. Bioaccumulation of toxic metals in an organism with time

An illustration below the text depicts, from left to right, algae, a mayfly larva, and a fish. Each organism depicted has orange dots in or on it, with arrows pointing to the next organism.

3. Ecosystem contraction, resulting in loss of habitat due to degraded conditions

An illustration below the text depicts the same as above, but with "not" icons (circles with slashes) over the larva and the fish.

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“This kind of acid rock drainage is what you expect to see at a mining site, not in the remote and pristine Brooks Range.”

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In recent years, Arctic air and ground temperatures have abruptly increased compared to the preceding 30-year record, exceeding the freezing threshold for near-surface permafrost. Thawing permafrost can alter stream chemistry through changes in watershed vegetation, soil, and topography. More water reaching deeper in the ground unlocks minerals and exposes them to microbes and weathering. The weathering of sulfide minerals like pyrite releases acid, iron, sulfate, and potentially toxic metals to streams and rivers, which discolor the water and displace fish. “This kind of acid rock drainage is what you expect to see at a mining site, not in the remote and pristine Brooks Range in Alaska<,” said Brett Poulin, assistant professor in Environmental Toxicology, University of California-Davis, and a co-author of the paper.

Why This Is Important

Arctic rivers support many types of fish used by subsistence harvesters, recreational anglers, and commercial fisheries. These fish are already suffering the effects of climate change. Metal release due to thawing permafrost is one more stressor. When the Akillik River changed color from clear in 2017 to orange in 2018—and when the stream pH suddenly dropped and metal concentrations increased—it completely lost two fish species, juvenile Dolly Varden and slimy sculpin. Aquatic invertebrates also declined.

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Impaired streams from metal contaminants are a serious problem for nearby villages.

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Although arsenic and lead did not exceed EPA or World Health Organization recommendations in study streams, the researchers found concentrations of cadmium, nickel, and manganese that did. Impaired streams from metal contaminants are a serious problem for nearby villages. Metal contamination from this naturally occurring leaching process extends the problem to a large area.

Arctic rivers are ever-changing ecosystems. The specific causes and long-term consequences of their impairment remain to be discovered. “We are still learning about what makes certain rivers vulnerable to rusting, as the underlying causes are complex,” said O’Donnell. But this study will help scientists, parks, and communities understand why rivers are turning orange. It gives us a better idea of how big the problem is and how far it has spread. With further research, it could also help determine how long the discoloration might last.

Acknowledgments

Research on rusting rivers was funded through a grant from the U.S. Geological Survey-National Park Service Water Quality Partnership Program, and through ongoing support from the National Park Service Inventory & Monitoring Division.

About the author

^{Nina Chambers is a science communicator with the Alaska Region of the National Park Service. She is the managing editor of Alaska Park Science. Image courtesy Nina Chambers.}