Now-Annual Glacial Outburst Floods On Mendenhall Glacier Near Juneau Growing Steadily In Volume

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.

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https://www.alaskapublic.org/2020/07/28/a-glacial-dam-near-juneau-could-burst-any-time/