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Rockslide shatters Mendenhall Lake ice

Naturalists, experts advise caution

Posted: December 6, 2013 - 1:02am
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From Nugget Falls the recent landslide on the slopes of Mount Bullard is not easy to distinguish. But the remnants of ice chunks created by the slide are barely visible.   Mary Catharine Martin | Juneau Empire
Mary Catharine Martin | Juneau Empire
From Nugget Falls the recent landslide on the slopes of Mount Bullard is not easy to distinguish. But the remnants of ice chunks created by the slide are barely visible.

The Forest Service advises caution for those traversing Mendenhall Lake after a Mount Bullard rockslide last week shattered the lake’s frozen surface.

“It’s a really interesting thing,” Laurie Craig said. “We warn people about ice safety all the time, but this is something I hadn’t really thought about — a rockslide doing what it did.”

Craig, the lead naturalist at the Mendenhall Glacier Visitor Center, has watched the landscape surrounding the glacier change with the seasons for many years. She’s also seen natural events, most often a winter glacial calving event, turn the frozen lake into one big “Slushie.”

But this was a new one for Craig who said the rock came down a third of the way between Nugget Falls and the terminus of the glacier, leaving floating debris — trees and branches — in the lake, and a pale, “fresh-looking” section of rock on the mountain above the lake. It also sent a wave across the lake, breaking up ice on the peninsula on the other side.

UAS Professor of Geology Cathy Connor said the rockslide may have been caused by freezing and thawing temperatures acting “like a jackhammer.”

Frequently in Southeast Alaska, water gets in cracks in the rock. It then freezes, melts and freezes, loosening that rock, she said.

Areas the USFS regularly recommends avoiding are near the glacier, icebergs and any areas of open water flow.

“The key is that just because the glacier is retreating, it doesn’t mean it’s not flowing downhill every minute,” Connor said. “There’s still a net force pushing downhill against the lake ice … the seam between the glacier front and the lake ice is constantly being pushed and shoved.”

Connor and Craig also advise people to avoid the icebergs, which are often frozen into the lake’s surface.

But area users are still taking chances. Craig said she can see footpaths going to the lake’s current icebergs.

“They’re still melting in the water underneath,” she said. “But they are magnets for people … We can see from the patterns on the lake ice that a lot of people have been going out on the icebergs.”

And while these masses of ancient ice are certainly beautiful, they are far from stable.

“Because they’re so bottom heavy, they can do stuff under the lake that loosens the grip of the lake ice around them,” Connor said. “They roll and move still, because they have such mass below the ice.”

Craig also said she saw a “shooter” recently. A shooter is a piece of the glacier that calves beneath the surface of the lake. When it disconnects from the glacier’s terminus, it shoots up to the surface and breaks through the ice.

The USFS stresses that ice reliability on the lake is unpredictable, regardless of the temperature or thickness.

For more information on how to recreate safely in the vicinity of the Mendenhall Glacier, the public is invited to attend a half-day ice safety training, hosted by Capital City Fire and Rescue and the Forest Service, on Jan. 11 at the Mendenhall Glacier Visitor Center. The event is tentatively planned to run from 1-4 p.m. Besides ice safety, there will also be demonstrations; the event is free and open to the public.

 

• Contact Outdoors reporter Mary Catharine Martin at maryc.martin@juneauempire.com.

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