Why Arctic rivers are turning orange

Across Alaska’s Brooks Range and other parts of the Arctic, dozens of once-clear streams and rivers are turning bright orange. Scientists are documenting this accelerating environmental phenomenon, which they say is directly tied to climate change. The change, documented extensively in NOAA’s 2025 Arctic Report Card, was found in more than 200 watersheds in Arctic Alaska over the past decade. 

As temperatures rise, the region is experiencing the thawing of permafrost that has been frozen for thousands of years. The result: rivers that appear to be rusting from the inside out, a transformation so dramatic it is visible from space.

Researchers describe the phenomenon as similar to acid rock drainage that occurs in mining regions. However, unlike discoloration from industrial pollution, this “rusting” reflects fundamental chemical shifts in watershed systems as previously frozen ground and minerals become exposed to oxygen and water. 

The phenomenon represents not just a change in appearance but a shift in water chemistry, marked by increased acidity and elevated levels of iron and other metals, carrying significant implications for local ecosystems and communities. Scientists say the geographic spread of these orange rivers has been both surprising and difficult to reverse once initiated.

Key findings:

• Clear climate driver: The spread of rusting rivers coincides with accelerating permafrost thaw and record Arctic temperatures, underscoring that this is a climate-driven shift rather than a localized anomaly.
• Geochemical mechanism: As frozen ground thaws, sulfide- and iron-rich soils are exposed to oxygen and flowing water, triggering acid rock drainage that releases iron, acidity and dissolved metals such as manganese and nickel into rivers.
• Water quality concerns: In some affected systems, acidity and metal concentrations exceed U.S. Environmental Protection Agency chronic aquatic exposure criteria and, in certain cases, World Health Organization drinking water guidelines.
• Ecosystem disruption: Researchers have documented declines in aquatic biodiversity, including the loss of fish species such as Dolly Varden and slimy sculpin in at least one monitored watershed.
• Emerging circumpolar pattern: Similar river discoloration has been observed in parts of northwestern Canada and Siberia, suggesting the phenomenon is expanding across the Arctic as thaw intensifies.
• Carbon feedback risk: Permafrost thaw mobilizes stored organic carbon and greenhouse gases. Scientists warn that incomplete accounting of these releases could lead climate models to underestimate future warming.
• Long-term permafrost loss: Research indicates that under high-emissions scenarios, up to 93 percent of the world’s near-surface permafrost (the upper three to four meters) could thaw by 2100, destabilizing infrastructure, disrupting ecosystems and amplifying carbon release. Only limited areas of Siberia, Canada and Greenland are projected to retain this frozen layer.
  

The emergence of rust-colored rivers in the Arctic is more than a striking visual. It is a reflection of broader, climate-mediated changes in freshwater systems integral to Arctic ecology, human health, and long-standing cultural practices. For communities reliant on local watersheds for drinking water, subsistence fishing, and traditional livelihoods, the shift toward acidic, metal-rich rivers may demand new water treatment strategies and adaptive resource management.

Ecologically, altered chemistry threatens fish populations and invertebrate communities that underpin food webs. The phenomenon is a stark indicator of how rapid permafrost thaw, an irreversible process on human timescales, is reshaping northern landscapes and freshwater chemistry, with consequences likely to intensify as warming continues.

Read the NOAA Report Card here.