Landslide above Portage Lake is on geologists' radar as tsunami threat

A landslide near the southern end of Portage Lake is on geologists' radar as a tsunami threat. There are concerns debris from the slide could fall into the lake, creating a tsunami that could be 50 feet tall near the area of the Begich, Boggs Visitors Center.

"There's a lot of things we don't know," Alaska geologist Bretwood "Hig" Higman said. "And some of this is because reality could play out in various different ways and some of this is because the science isn't there."

The slow-moving landslide is above the front of the glacier, he said.

"If you were looking at the glacier on the right, this slope on the right has started crumbling, and a lot of it is moving slowly downward," he said.

"Parts of it are falling off abruptly but they've been small parts so far. And the worry is that a big chunk could come loose. And if that hit the water, it could generate a tsunami in the lake that could be very large."

There's still a lot to learn about the situation, Hig said. He has been studying it since 2022.

"It's the sort of mass that could produce a tsunami that would be many hundreds of feet tall in some places and certainly tens of feet tall throughout the margin of the lake. And so, that's why we're really worried about it -- it is big," he said. "Is it record-breaking in the world? Definitely not. Is it bigger than most landslides you encounter? Definitely."

Alaska Landslide Hazards Program Manager Mort Larsen agrees that monitoring is crucial.

"That's a major corridor system, it's a recreational system so it's definitely something that people need to pay attention to, something that needs to be observed and monitored to a certain degree," Larsen said, "so the right information gets to decision makers so they can make decisions on relaying that information to public."

Hig mentioned further work needed for accurate tsunami modeling but acknowledged risks exist.

"Reality is that visitor center is at risk and valley is at risk. I have high confidence in that," Hig said.

"I wish I could say with great confidence exactly how much risk there was... We've done some initial analysis trying to figure that out," Hig added regarding his research efforts.  

He also expressed concern about nearby infrastructure like tunnels into Whitter alongside highways/rail lines/power lines being vulnerable too:

'Some very bad cases would involve something sweeping all way down valley endangering everything Portage Valley,'

. 'Some models we've run show wave reaching tunnel's portage-end; because downhill path exists-water may flow through.'"

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