Story last updated at 9/2/2009 - 2:32 pm
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.
The slump near Selawik is unusual both for its size and because it may affect the villagers in Selawik, about 175 miles downstream. People in Selawik rely on sheefish as one of their food sources. Sheefish, which can live to be 30-years-old and weigh as much as a plump Yukon River king salmon, scatter their eggs between gravel stones of the river bottom in a preferred stretch of river downstream of the slump. Biologists think the sediment the slump is leaking into the river may fill in those nooks and crannies.
Some generations of sheefish may already be affected, but biologists don't yet know, because those long-lived fish don't return to breed for at least seven years. Because the slump happened around 2004, it will probably be a few more years before biologists can tell that the added sediment might have affected them.
"If we were to see any downturn, it wouldn't be until (about 2014)," said Ray Hander, a biologist with the U.S. Fish and Wildlife Service in Fairbanks who is studying the Selawik River sheefish. When he checked last fall, there was still some gravel showing on the sheefish spawning grounds, about 25 miles downstream of the slump, Hander said.
Like Hubbard Glacier threatening the lifestyles of Yakutat residents who guide for steelhead in the clear Situk River (which would turn cloudy if the glacier blocked Russell Fiord), Selawik Slump is another example of how thawing Alaska ice could change life in a community.
"It's something that's always been there," Hander said about the subsistence relationship between villagers in Selawik and sheefish. "It's not the only fish they seek out, but sheefish are available year-round - jigging under the ice, gill nets, hook and line. They're a food source people depend upon."
The Selawik Slump will endure for decades, scientists think, though its sediment load to the river will wax and wane. And though the feature is unusual in its size, there are thousands more out there in northern Alaska.
"We find them in the landscape where they've occurred in the past; places we see them initiating now are near older scars," Crosby said. "The process has happened for thousands of years. It's just accelerating now."
This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute.