Why are Alaska’s rivers turning orange?

Minerals are seeping out of the thawing ground in remote parts of the state.
Laura Baisas Avatar
An aerial view of the rust-colored Kutuk River in Gates of the Arctic National Park in Alaska. Thawing permafrost is exposing minerals to weathering, increasing the acidity of the water, which releases metals like iron, zinc, and copper.
An aerial view of the rust-colored Kutuk River in Gates of the Arctic National Park in Alaska. Thawing permafrost is exposing minerals to weathering, increasing the acidity of the water, which releases metals like iron, zinc, and copper. Credit: Ken Hill/National Park Service

Thawing permafrost may be turning clear water in some of Alaska’s remote watersheds into a dusty, murky orange. As the frozen layer of soil melts, minerals become exposed and seep into the water, according to a study published May 20 in the Nature journal Communications: Earth and Environment

“The more we flew around, we started noticing more and more orange rivers and streams,” study co-author and National Park Service ecologist Jon O’Donnel said in a statement. “There are certain sites that look almost like a milky orange juice. Those orange streams can be problematic both in terms of being toxic but might also prevent migration of fish to spawning areas.”

Unstable ground

Permafrost is rock or soil containing ice that remains frozen for two or more years. According to the Environmental Protection Agency (EPA), about 80 percent of the ground in Alaska has permafrost underneath it throughout the year. The state is also warming two to three times faster than the global average, thawing some of the state’s plentiful permafrost.

[Related: Earth’s cold storage is melting. Here’s what’s oozing out.]

As ice-filled permafrost begins to thaw, it can turn into mud that can’t support the weight of the soil or vegetation above it. Human-built infrastructure including buildings, homes, pipes, and roads may also be damaged. It can also expose organic matter, including the remains of dead plants, that can release methane and carbon dioxide into the atmosphere as the ground thaws. Long-dead viruses or bacteria that could potentially threaten human health can even be released. 

‘We can see them from space’

In 2018, O’Donnell first noticed a river that appeared rusty, despite having appeared clear the year before. To look closer, O’Donnel and the team on this study documented and sampled some of the impaired waters. They looked at 75 locations across a section of northern Alaska’s Brooks Range that was about the size of the state of Texas. 

“The stained rivers are so big we can see them from space,” study co-author and University of California, Davis geochemist Brett Poulin, said in a statement. “These have to be stained a lot to pick them up from space.”

Poulin observed that the staining in the water appeared similar to what happens during acid mine drainage–when highly acidic water is moved from metal or coal mines. However, there were no mines near any of the impaired rivers, which include the Salmon River and other federally protected waters.

Their hypothesis is that when the permafrost melts as the temperature warms, the stored minerals inside of the frozen ground are released. Metal ores that were once locked up were then exposed to water and oxygen, releasing acid and metals into the river. 

“Chemistry tells us minerals are weathering,” Poulin said. “Understanding what’s in the water is a fingerprint as to what occurred.”

Poulin and UC Davis Ph.D. candidate Taylor Evinger analyzed initial samples that were collected in June and July 2023 and then took their own in August. 

They found that some samples from the impaired waters have a pH of 2.3. The average pH for these rivers is 8. This change in pH indicates that the silfide materials are weathering and making the water more acidic. They also measured elevated or high levels of iron, zinc, nickel, copper, and cadmium.

“We see a lot of different types of metals in these waters,” Evinger said in a statement. “One of the most dominant metals is iron. That’s what is causing the color change.”

Implications from fisheries stocks to drinking water

The team is currently in the second year of a three-year grant aimed at understanding what is happening in the waters, determining what other areas could be at risk, and assessing what it means for fishing stocks and drinking water. 

According to the team, the problem is growing and affecting habitat, water quality, and other ecological systems. It is degrading otherwise healthy habitats and could threaten the safety of drinking water in rural areas. 

An aerial view of the Kutuk River in Alaska’s Gates of the Arctic National Park, where a portion of the water is rust-stained. CREDIT: Ken Hill/National Park Service.

“There’s a lot of implications,” O’Donnell said. “As the climate continues to warm, we would expect permafrost to continue to thaw and so wherever there are these types of minerals, there’s potential for streams to be turning orange and becoming degraded in terms of water quality.”

[Related: Wildfires are thawing the tundra.]

More detailed work is needed to better understand the problem and to see if rivers and streams can rebound after colder weather restores permafrost. Satellite images dating back to 2008 show that the waters have been changing color for well over a decade and could only be growing over time. 

“The issue is slowly propagating from small headwaters into bigger rivers over time,” said O’Donnell. “When emergent issues or threats come about, we need to be able to understand them.”