Why are Alaska’s rivers turning bright orange?

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