Scientists explain why the Arctic's rivers are turning rust orange
A new UC Riverside and University of Alaska study describes two mechanisms by which thawing permafrost loads Arctic rivers with orange iron particles.
- A UC Riverside and University of Alaska team has identified two mechanisms turning Arctic rivers rust orange.
- One is acid weathering of rock, the other is microbial iron reduction in spreading wetlands.
- Fine iron particles travel more than 100 kilometres and smother algae, insects and fish.
- The work appears in the journal Communications Earth & Environment.
A team from the University of California, Riverside (UC Riverside) and the University of Alaska has published a study explaining why a growing number of Arctic rivers are taking on a rusty orange colour. The paper appears in the journal Communications Earth & Environment, with a summary released on 1 June 2026. The research builds on already documented contamination of waterways in Alaska’s Brooks Range, where some once clear rivers turned cloudy and stained within a few seasons.
According to the team, the colour does not come from a single source. The scientists describe two separate mechanisms that release iron from thawing ground at the same time. Both are driven by the same background process, the thaw of permafrost, the permanently frozen layer that for centuries kept minerals and organic matter locked away from water and air.
Which two mechanisms turn the rivers orange?
The first mechanism is acid weathering of rock. When water reaches pyrite, the mineral also known as fool’s gold, the pyrite breaks down into iron and sulphur. The reaction produces sulphuric acid, which then pulls other toxic metals out of the surrounding stone. The freed iron, once exposed to air, oxidises into tiny rusty particles that tint the water orange.
The second mechanism plays out in wetlands that expand as the permafrost retreats. There, microbes carry out iron reduction, a chemical process that converts the metal into a form that dissolves in water. When that water drains into a river and meets oxygen, the iron oxidises again and settles out in the same orange form. Two different routes, then, lead to the same result.
- Pyrite meeting water yields iron, sulphur and sulphuric acid
- The acid leaches additional toxic metals from rock
- Microbes in wetlands convert iron into a dissolved form
- In both cases iron settles out as rust particles in air
Why the damage runs deeper than the colour
The fine iron particles do not stay where they form. According to the study, they can remain suspended and be carried downstream for more than 100 kilometres. Along the way they smother bottom algae, disrupt aquatic insect populations and clog the gills of fish, weakening the food chain in these cold Arctic rivers from the ground up.
The researchers also flag a delayed effect that makes the problem hard to forecast. Iron freed during one summer thaw does not necessarily wash into the river straight away. Some of it lingers in the soil and is only flushed out by the following year’s rain and melt. That means the consequences of one warm summer can surface a full season late.
The colour of these rivers is only the surface sign of deeper chemical change in Arctic soils, and the fine iron particles can travel more than a hundred kilometres downstream, reshaping life in the water.
The limits of the work are worth stating. The study describes the mechanisms and tracks them in Alaska’s Brooks Range, but it does not claim that every rusting Arctic river shares the same cause, nor that the process runs equally fast everywhere. The scale of the phenomenon across the whole far north remains to be measured. What the paper does show clearly is that permafrost thaw is already altering the chemistry of surface water. How quickly do you think warmer winters could push the same kind of change into rivers closer to home?
