It’s been over a year since a landslide devastated the Wrangell community, killing six people. Last month, geologists presented their work from a visit to the slide over the summer.
Margaret Darrow, a professor of geological engineering at the University of Alaska Fairbanks, has been studying the landslide with her colleagues. She said their research is still in the works, but they’re inching closer to answers.
“My greatest hope is that whatever we find from this work will be able to tell the community of Wrangell why this slide might have happened, where it happened, where other slides could happen, so that you could use it in community planning,” Darrow said.
So far, they’ve found that the slope held an unusual amount of loose material waiting to be set in motion – and that the muskegs on top of the ridge may have played a role in doing that.
A surprisingly large volume of loose material
Their soil and rock samples are still being processed, but so far the researchers can say that the soil where the landslide happened is unusually loose. It sits on top of glacial sediment, which acts like a barrier against water. That could have been a huge contributor to the slide.
Annette Patton, a geologist at Oregon State University, said there was a surprisingly large volume of the loose material.
“A lot of the hill slopes around Southeast Alaska have really thin soils because it’s just so steep and material can kind of slide right down,” she said. “But part of why this landslide was so large is because there was actually a very anomalously thick layer of very loose material.”
She said the team looked at records of old landslides in the area. They think the 2023 landslide — which took out about 200 trees — happened right below what they think was an older slide.
“We just wanted to show this as an example of the fact that there is a lot of movement and activity that’s happened on this hill slope since the last glacial maximum,” Patton said. “And there’s a lot that we don’t understand about exactly how that might play out.”
She said there was a large storm the day of the landslide of a type known as an atmospheric river, but it wasn’t out of the ordinary for Southeast Alaska.
“Something notable here is that it wasn’t a really extreme storm,” Patton said. “It had a return period of about one year. So it’s like a big winter storm, but the kind of storm that maybe happens at least once a year.”
But the rainfall monitoring was done only at the airport, 11 miles away from the landslide and at sea level. That monitoring system recorded a little over an inch of rain during the six hours before the slide. Some Wrangell residents said they recorded three inches of rain that day, closer to the slide.
“These are really common types of storms,” Patton said. “Most landslides are triggered by atmospheric rivers here in Southeast Alaska. But not all atmospheric rivers trigger landslides.”
But Patton said there was a lot of water on the slope — and it mobilized all of that loose material.
More than twice as big as any known Wrangell slide
Her colleague, Josh Roering, a professor at the University of Oregon, said that the U.S. Forest Service started paying attention to landslides in the 70s, and a lot of their research came out of Wrangell and Prince of Wales.
“You’re really in the epicenter of a lot of amazing discoveries that have continued to affect how we think about these processes that led to the Forest Service mapping landslides every year across the region,” he said. “The map for Wrangell includes 256 landslides.”
He said it’s helpful to look at all the surrounding landslides in order to contextualize the massive one from 2023.
“It was more than twice as big as the next biggest slide that’s happened on Wrangell,” he said. “This was truly an extreme, anomalous event in terms of size, compared to what has happened here before. So this really led us to ask the question, ‘What is so different about that setting that allowed it to behave so differently?’”
He said they were also able to use the LIDAR data from the State Division of Geological and Geophysical Science. The department surveyed the area months before the landslide happened. The department also surveyed the area after the slide.
Roering said the first thing they noticed was some large ledges, or steps, exposing the bedrock.
“These are really big steps,” he said. “Looking at it from the road does not prepare you for how big they are in person.”
He said the erosion wasn’t consistent throughout the slide — most of it happened at the steep bedrock steps. And they even found living blueberry bushes right below some of those cliffs.
Roering said that implies the slide came down and almost launched from one ledge down to the next — which would only be possible if the soil was liquefied. And that would take an enormous amount of water.
“This field work occurred in August of 2024 so about six, seven months ago, and it was still really, really wet,” he said. “It hadn’t rained in a while, weeks before we were there. Yet there’s still what we call seepage – a lot of drainage from the landscape above the scarp that was coming into this side.”
The muskegs on the ridgetop
Roering said they wanted to know where the water came from, so they used previous LIDAR data to find the path from the top of the ridge to the beginning of the slide.
“As we follow these flow paths, they go up another set of bedrock ledges, and then they get up on top of the ridge,” he said. “We spent a lot of time up here on this ridgetop muskeg, trying to imagine the plumbing system for how this works, how the water goes up and down, how it spills out in some places and not others.”
He said they put in hydrologic sensors that test water levels that will help them understand when and how much water gets channeled down from the ridgetop muskegs. The researchers will get the sensors 14 months after installing them. They’ll see if the water levels remain constant or fluctuate a lot during the time period.
Roering said the muskegs only form here in areas of flat land — not where the ridge is too steep. And they can hold a lot of water and channel it downhill.
“In some ways, having channels to take that water out is a good thing, but in a lot of cases, having channels funnel water to one location where there’s a lot of loose material is obviously a really bad thing,” he said.
Roering wrote in an email that the likelihood of another landslide happening in the same area is low because the scar left behind doesn’t have much material left to be mobilized.
The researchers also gave tips for recognizing when a landslide might be about to happen — like sudden changes in water flow or color. Another indicator would be sound — some have compared it to a falling jet or a tornado. The researchers said that once people hear a landslide, they only have moments to get out of the way.
They also encouraged people to pay attention to weather forecasts, as landslides usually happen during intense rainfall. People can report a landslide on Alaska Landslide Reporter, an app that the state of Alaska recently released.
Copyright 2025 KTOO