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Jökulhlaup sparks flood warning in area surrounding the Mendenhall Glacier

Mendenhall Glacier. (07/31/2020)
Mendenhall Glacier. (07/31/2020)(KTUU)
Published: Jul. 31, 2020 at 6:59 PM AKDT
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JUNEAU, Alaska (KTUU) - A torrent of water is rushing underneath the Mendenhall Glacier, sparking a flood warning in the area surrounding Mendenhall Lake and River.

The glacier dam release is known as a jökulhlaup. The Suicide Basin which sits around one mile up the Mendenhall Glacier fills with around 10,000 Olympic-sized swimming pools worth of water.

“Once that water reaches a certain level, it actually pushes up the main trunk, the main sum of the glacier,” said Aaron Jacobs, a senior hydrologist with the National Weather Service in Juneau. “And all that water is able to rush up underneath the glacier.”

The water causes the Mendenhall Lake to rise and the Mendenhall River to turn into a raging torrent, threatening nearby homes. Some homes have been inundated in previous years when the glacier dam released.

Mitigation efforts have been taken after the biggest water rise in 2016. One homeowner raised their house by 6 feet to avoid the floods, Jacobs said.

On Thursday evening, the National Weather Service released a flood advisory, saying the river would rise by 10.2 feet, cresting on Friday evening.

The now annual glacier dam release is monitored by the National Weather Service and has seen the potential for significant flooding since 2011. Before then, Suicide Basin filled with ice. Jacobs says a warning climate means that no longer happens.

The receding glacier also means in recent years that water spills over the top of Suicide Basin, reducing the amount of water that spills downstream.

“It’s changed a lot,” said Frankie Pillifant, an earth scientist and downtown Juneau resident, who came out Friday to see the water rise from underneath the Mendenhall Glacier.

Pillifant said she has seen the glacier slowly creep back in recent decades. Less than 30 years ago, its terminus sat in front of Nugget Falls, now it’s far back from there.

Earlier in the week, there were concerns that the 2020 jökulhlaup could cause significant flooding. A rainy summer in Southeast Alaska had seen the Mendenhall River at bankfull.

The sun came out in recent days and the water level dropped by half. “Good thing now is that we’re sunny and dry,” Jacobs said on Wednesday.

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