Scientists, emergency managers and Juneau residents are bracing for an event at the Mendenhall Glacier that could flood a nearby lake and river again. The now-yearly phenomenon is caused by climate change.
The important vocabulary word for this story is jökulhlaup — pronounced yo-KOOL-lahp — an Icelandic word for a glacial dam release or flood.
“The famous examples are from Iceland, where they have really massive outburst floods,” said Jason Amundson, who studies and teaches about glaciers as associate professor of geophysics at the University of Alaska Southeast. “Those are geothermally driven.”
“So, you have a glacier sitting on a volcano and you get this sort of subglacial lake sitting there,” said Amundson. “And then, that can drain catastrophically. That’s definitely affected the road system and knocked out two pretty big bridges.”
Amundson said jökulhlaups in Alaska and off the Mendenhall Glacier happen a little differently.
Ice melt and rainfall accumulate every summer in Suicide Basin, a gigantic bowl-shaped valley adjacent to the glacier and about two miles up. The basin was carved out of the granite by a much smaller glacier, which retreated in the last few decades because of climate change. So much water builds up in the basin that the side of the Mendenhall Glacier starts floating. The water’s immense pressure also creates cracks in the ice called conduits.
“The water starts melting the conduit bigger and bigger,” explained Amundson. “So, as the water flows through, the discharge is low. But as it’s flowing, it melts the channel and the channel gets bigger which makes the discharge get bigger which makes the melt rate higher until you get this rapid growth.”
All that water dumps into Mendenhall Lake and Mendenhall River in a matter of hours.
Amundson said there are other glaciers around Alaska that have similar releases.
Previous flooding in Juneau has covered a road, inundated a campground, damaged at least one house and eroded the riverbank backyards of other homes.
That’s why Jamie Pierce of the U.S. Geological Survey attached monitoring equipment to the high-angle rock walls of Suicide Basin in 2012.
“(It) worked for a couple of years, but then the ice just sheared the cable,” Pierce laughs. “I didn’t think the ice would cream it, but it did. It kind of showed the power of this stuff. I mean, really!”
New underwater instruments, without the metal cabling, were installed last year.
The UGS shares that data with researchers and other agencies so they can warn the public.
Aaron Jacobs, a hydrologist with the National Weather Service in Juneau, says they’ve documented jökulhlaups on the Mendenhall every year since 2011.
“Typically see one to two a year,” Jacobs said. “But there’ve been years in which we’ve seen multiple glacial dam releases, but on a smaller magnitude or size.”
In the past, all of the water would flow out of Suicide Basin under or through the glacier. But scientists have noticed something new. The side of the glacier, which acts as a giant ice dam, is thinning and getting lower because of climate change. Water spilled over that ice dam for the first time last year.
Then, scientists noticed in mid-July this year that the basin’s rising water caused a portion of that ice dam to calve or break off.
“It pushed a whole bunch more ice into the basin and made for some very huge, monster icebergs,” Jacobs said.
Water is already spilling over the ice dam. And the lake below the glacier is already approaching flood stage from rain water.
The big jökulhlaup could happen anytime.
Scientists say it’s tricky to predict how big the flood will be this time because the basin and the amount of water inside it are constantly changing.