Why are Alaska’s rivers turning bright orange? Scientists have a theory.

In the summer of 2019, ecologist Patrick Sullivan and a Super Cub plane pilot navigated over the narrow valleys of the Brooks Range in northern Alaska, winding toward the remote headwaters of the Salmon River. Sullivan was studying the slow advance of trees into what had been tundra, a sign of the rapidly changing climate, but soon discovered something far more surprising.

Sullivan was expecting a clear, cold river artery with blue-green pools, and had even brought his fishing rod. So he was shocked to see turbid water and banks stained a fluorescent orange. “It looked like sewage,” he recalls. As the research team finished their sampling and paddled downstream in pack rafts, the cloudy, tangerine water persisted. Along the river, they floated past several emaciated bears. At one quiet eddy, a particularly thin bear crept near, its dark eyes fixed on them in a way that made Sullivan uneasy. He wondered if the river’s degraded state was suppressing the fish populations, potentially reducing bears’ food sources. “I started to feel like what we were seeing was an example of ecosystem collapse,” he says.

On a later trip, Sullivan took water samples and found strangely high concentrations of iron and heavy metals like cadmium, aluminum, and in some cases, zinc. By then, Sullivan realized the problem extended far beyond the Salmon: Bush pilots reported the same rusty color in dozens of rivers across the region, spanning an area roughly the size of Nebraska. Researchers around the world have begun to document similar changes in waterways near permafrost.

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