Thaw scars widespread across
By NED ROZELL
October 01, 2009
One month ago, I wrote about a dramatic landscape feature in
western Alaska called the Selawik Slump. The slump, caused by
thawing permafrost, looks like a bomb crater leaking mud from
the boreal forest into a clear northern river. There are dozens
of them in northern Alaska, though none as big as the one on
the Selawik River.
There are also many of these beacons of change in the Yukon Territory,
according to Doug Davidge of Whitehorse, who read the Selawik
column in the Yukon News. A few years ago Davidge was flying
over the Peel River country east of Eagle Plains for work when
he saw a gaping wound on a hillside. Scientists once described
these features as "tundra mudflows." They now call
them retrogressive thaw slumps.
A double progressive
thaw slump on both sides of the Caribou River, which flows into
the Peel River in northern Yukon Territory. It is located at
about the same latitude as the Selawik Slump.
Photo by Doug Davidge.
A retrogressive thaw
slump on the Bonnet Plume River, which flows into the Peel River
in northern Yukon Territory. The top of the feature is about
a half-mile-wide. The mudflow extends more than one mile.
"We flew over some very dramatic looking retrogressive thaws,
and we could pick out other ones as we flew along," he said.
Davidge snapped a photo of the largest thaw slump he and the
pilot noticed, near a drainage called Bonnet Plume. Though not
as large, it looks similar to the feature that is now clouding
the Selawik River. Retrogressive thaw slumps form when warm air
eats at permafrost that contains large bodies of ice. The features
often develop when a river cuts into a frozen bank on an outside
As ice and frozen soil thaw back into a bank or hillside, more
and more of the ground collapses in on itself, leaving a crater
of churned soil backed by a steep, frozen headwall. That wall
retreats over time, mud flows downhill and often into rivers,
and the slump grows.
Though it's not part of Davidge's job for Environment Canada,
the giant scar on the landscape intrigued him enough to send
him to Google Earth on his own time to search for more of the
characteristic scoops missing from hillsides along the remote
Peel River drainage. He found a lot of them.
Photo by Doug Davidge.
"I don't have the total count in front of me," he said
from his office in Whitehorse. "I keep adding them to the
list, but it's probably in the range of 200 or so . . . There
are many other huge ones out there."
Yukon Geological Survey Geologist Panya Lipovsky has studied
the retrogressive thaw slumps for a few years in central and
southern Yukon. One of the more interesting ones she's encountered
is 60 miles north of the town of Ross River on the South MacMillan
River; it's known as the Surprise Rapids Slide.
"That's the biggest one I'm aware of," she said, also
from her office in Whitehorse. "It's about (2,300 feet)
wide and has traveled up to about (one-and-one-half miles from
the hillside to the river). It may have been triggered by a forest
fire in the 1870s and still hasn't stabilized in more than 100
years. The headwall keeps melting back."
Some of the slumps she has observed are recent and some date
back more than a century. They may be related to forest fires,
intense rainfall, meandering rivers, and/or warming air temperatures,
she said. One of the more active periods for the Surprise Rapids
Slide happened in the 1940s, when the central Yukon had record
high early summer temperatures, according to Brent Ward of Simon
On the South MacMillan
River in the Yukon Territory, the
Surprise Rapids retrogressive thaw slump may have started flowing
forest fire in the 1870s.
Photo courtesy Panya Lipovsky.
Lipovsky knows of slumps that are eating into hillsides at the
rate of 30 feet per year. Some have altered the flow of rivers
and have clouded them up with their runoff, but she hasn't heard
of any troubling people in the Yukon, as the Selawik Slump might
be affecting the people of Selawik should cloudy water damage
sheefish spawning grounds.
There aren't many scientists who are studying these eye-catching
northern expressions of permafrost lost, but thaw slumps may
draw more attention as they continue to express themselves on
the less-frozen northern landscape.
"We've only just started keeping an inventory of these things
in the last couple years," Lipovsky said.
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.
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