In a remote fjord of Prince William Sound, a mountainside above a retreating glacier has been slowly loosening its grip on the rock beneath it. Geologists have flagged the slope at Barry Arm, roughly fifty miles northeast of the small port town of Whittier, as one of the more closely watched landslide hazards in Alaska. The worry is not the landslide alone but what a sudden collapse into deep water could trigger: a surge known as a megatsunami, a wave whose initial height can dwarf the tallest buildings on Earth before it races down the fjord.
How a falling mountain makes a wave
A megatsunami is a different beast from the ocean-crossing tsunamis spawned by undersea earthquakes. Instead of the seafloor lurching, an enormous mass of rock plunges into a confined body of water and displaces it violently, throwing water up the opposite shore and sending a wall of it surging outward. In the narrow, steep-walled setting of a fjord, that energy cannot spread and dissipate the way it would in the open sea, so the wave stays tall and concentrated. The single most cited example is Lituya Bay, also in Alaska, where a 1958 rockslide launched a wave that stripped forest from a slope more than seventeen hundred feet above the waterline, a scale far beyond any skyscraper.
Why the Barry Arm slope is unstable
The trigger at Barry Arm is the long retreat of the glacier that shares the fjord’s name. For thousands of years the ice pressed against the base of the mountainside and helped hold it in place. As the glacier has thinned and pulled back, it has stopped buttressing the slope, leaving a vast body of fractured rock perched above the water with less support than it once had. Scientists tracking the site with satellite radar and other instruments have measured signs that the slope moved substantially over past decades, and a state hazard summary from the Alaska Division of Geological and Geophysical Surveys describes the feature as a large, slow-moving landslide capable of accelerating into a catastrophic failure under the wrong conditions.
What the modeling has estimated
The earliest published assessments of Barry Arm raised the alarm with dramatic numbers, warning that a rapid failure of the full slope could generate waves hundreds of feet high near the source. Later and more detailed modeling has moderated some of those figures. A subsequent analysis found that the largest plausible wave was smaller than the initial estimates, though still large enough to pose a serious hazard to anyone in the fjord and to communities and vessels around northern Prince William Sound. The updated work, summarized in a federal report on revised wave heights, underscores that even a downgraded scenario would send meaningful waves toward Whittier, where the tourism and fishing seasons draw people into the area.
Who is monitoring the slope
Because the consequences could reach populated areas, the site is watched by a coalition of agencies rather than a single office. The United States Geological Survey, the National Tsunami Warning Center, state geologists and university researchers have combined satellite observation, on-site instruments and hazard modeling to keep tabs on any movement. Their coordinated program, outlined by the agency’s landslide-hazards effort, aims to detect acceleration early enough to issue warnings. The goal is to distinguish the slow, ongoing creep that the slope already shows from the kind of rapid deformation that would precede a sudden collapse.
The current state of the hazard
As of the most recent assessments, the Barry Arm slope has not shown signs of large-scale active deformation that would signal an imminent, full-scale failure. That is reassuring but not a guarantee, because smaller portions of the unstable mass can break loose on their own and generate localized waves within the fjord. Landslide timing is notoriously difficult to forecast, since the final push toward failure can come from heavy rain, an earthquake, continued glacial thinning or a combination that no instrument can predict to the day. Monitoring therefore focuses on watching for change rather than naming a date.
Preparing communities for a low-odds, high-impact event
The challenge Barry Arm poses is a familiar one in hazard planning: an event that is unlikely in any given year but potentially devastating if it happens. Emergency planners have studied evacuation timing for Whittier and have weighed how quickly a warning could reach boaters, kayakers and cruise passengers who venture into Prince William Sound during the summer. The value of the monitoring network is measured not only in scientific data but in the minutes of warning it might buy. For now, the mountainside above the glacier holds, watched by satellites and sensors, a reminder that a warming climate can reshape landscapes in ways that create new dangers where a glacier once stood guard. The Barry Arm story shows how retreating ice can quietly transform a scenic fjord into a hazard that scientists feel compelled to monitor around the clock.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
More from Morning Overview
