Nearly four years after a devastating landslide took the lives of two residents, about 100 community members gathered at the Chilkat Center to hear geologist Mort Larsen address a familiar, yet pressing issue: landslide hazards.
The coastal Pacific Northwest has seen thousands of landslides, historic and modern – but there is more work to be done to understand the landslide frequency and intensity in Alaska, and what drives them.
Larsen, who works with the relatively new Landslide Hazards Program at the State of Alaska’s Division of Geological and Geophysical Surveys, offered insights on the science of landslides, what is being done to monitor them, and how the community can prepare for future events.
He began the meeting by acknowledging the 12 people who have died in Southeast Alaska slides over the last nine years. He cited events such as the 2015 Sitka landslide, the 2020 Beach Road landslide in Haines, and more recent incidents like the Wrangell landslide in 2023 and last month’s slide in Ketchikan.
“[They’ve] taken our friends, our family, and our loved ones from us,” he said.
The meeting was a reminder of the risks that come with living in a landscape prone to landslides, but it also provided the community with information about the efforts being made to map, assess and reduce those risks.
Understanding landslide risk
Larsen’s presentation underscored the importance of understanding the different types of landslides, from slow-moving debris flows to more sudden and catastrophic rockfalls. He said Alaska’s unique landscape: glaciers, thawing permafrost, and tectonic activity, creates ideal conditions for landslides.
He said some of the key factors that cause landslides are the heavy rain, seismic activity, retreating glaciers that destabilize slopes and thawing permafrost which weakens the ground.
“If things aren’t engineered properly, we see that can cause [landslides],” he said. “So humans can cause landslides.”
To better monitor these shifts, the Landslide Hazards Program has been using LiDAR (Light Detection and Ranging) technology, a remote-sensing tool that helps map the terrain. Larsen emphasized that while LiDAR is invaluable for identifying areas at risk, it’s not a crystal ball.
“LiDAR is not going to tell you when the next landslide is going to occur,” he said. “It’s not going to tell you what’s in the subsurface. But it’s at least a tool to give us an idea of models that we can put together.”
One significant advancement in landslide research has been the creation of susceptibility maps, which provide an overview of which areas are most at risk. The Landslide Hazards Program has been working on these maps for Haines, Cordova, Juneau and other vulnerable communities. The maps, which combine historical data with new technology, are designed to help city planners and residents make decisions about development and preparing for emergencies.
“We’re working on a statewide landslide-susceptibility map that’s pretty broad, but it gives you an idea of where a lot of these these issues that we’re dealing with in the state [are happening],” he said.
Old knowledge meets new science
Throughout the presentation, several in the audience listened intently – some taking notes and others sharing their experiences and concerns. One of them was Tim Ackerman, who offered to share stories passed down through oral history in the Chilkat Valley.
Ackerman shared stories of landslides, some pre-contact with Europeans. One such story involved a massive landslide at Chilkoot Lake that caused a wave large enough to wipe out a downstream community.
After the meeting, Ackerman said Tlingít oral history and stories of their valley are an entire body of knowledge that could help people answer the questions they have about where it’s safe to build, what causes landslides, and what they should be watching out for.
He said it’s a reminder that the mountains have always moved and people have to learn how to live with that.
His words underscored the importance of combining new science with indigenous knowledge and Larsen agreed. He said that while scientific tools like LiDAR and susceptibility maps are crucial, local knowledge can provide additional insights.
Larsen said after the meeting that he planned to follow up with Ackerman to learn more about the history of villages in the Chilkat Valley being taken out by landslides, and some of the other stories Ackerman wants to share.
Community Concerns
For many in the audience, the presentation raised new questions about the safety of their homes and infrastructure. One resident asked if there was any way to predict when a landslide might occur based on recent weather patterns. Larsen explained that while short bursts of heavy rain are a known trigger, it is difficult to predict exactly when a slope will fail.
Jill Nicolazzo, a state geologist who is in the hazard program with Larsen, said she looked at a few hundred landslide initiation points that were mapped in the Tongass and pulled the slope for each of those points.
“The average slope was between 36 to 39 degrees,” she said.
But even with that data, more study on things like soil composition are needed to develop more accurate predictions.
Larsen said the key to staying safe is to pay attention to the signs.
“You know, wet ground that’s typically not wet. New cracks and bulges in the ground,” he said.
Another one is flowing water that is suddenly running with a lot of sediment in it.
“These are signs that we’re trying to lead people to really kind of know your surroundings, know where you’re at,” he said. “You guys live in Haines, yeah. I’m sure you’re familiar with a lot of the areas that see activity.”