About five years ago, Kevin Fox was flying over the Selawik National Wildlife Refuge in northwest Alaska when he noticed the upper portion of the clear-running Selawik River looked cloudy. He traced the plume upstream and noticed what looked like a crater in the boreal forest; through a breach in its side, the crater was leaking a slurry of silt, gravel, and dirt into the river.

Fox, a pilot with the U.S. Fish and Wildlife Service in Anchorage, had noticed the Selawik Slump, as researchers are now calling the nine-acre permafrost-related scar that has changed the character of the Selawik River. The feature is growing every year, and it may threaten a rich sheefish run used by villagers from Selawik.


Ben Crosby of Idaho State University studies unique landforms, and he began travelling to and studying the Selawik Slump a few years ago, after his father-in-law, Caleb Pungowiyi of Kotzebue, told him about it.

"It's anomalously large," Crosby said by phone from Pocatello. "As far as we know it's the largest in North America."

Permafrost scientists have seen many smaller examples of the Selawik Slump around the north. They call them "retrogressive thaw slumps." These happen when ground that has been frozen for hundreds or thousands of years thaws on a slope, causing it to collapse. This exposes a new wall of frozen material, which then thaws to reveal another one (the back headwall of the slump is now about 80 feet high). In this way the feature eats its way into a hillside. It's as if a giant ice cream scoop removed a swath of forest and tundra.