Tracy Arm’s Post-Tsunami Landscape – Image for illustrative purposes only (Image credits: Pixabay)
Tracy Arm, a narrow fjord in southeast Alaska, changed dramatically in a single morning last summer. A massive landslide sent rock and debris plunging into the water, generating a tsunami that reached heights of nearly 1,600 feet on the opposite shore. NASA’s Landsat satellites recorded the transformation from orbit, showing green forest replaced by bare rock and brown debris across wide stretches of the shoreline.
The Morning the Slope Gave Way
On August 10, 2025, at least 64 million cubic meters of rock broke loose from the north wall of the fjord. The material hit the water with enough force to create a tsunami that raced across the inlet and climbed the far side. Trees and vegetation were sheared away up to 1,578 feet above sea level, leaving a clear “bathtub ring” visible from space.
The event also triggered a seiche, a back-and-forth sloshing of water that continued for more than a day. Seismic instruments around the world recorded signals from both the landslide and the seiche, with the initial slide registering the equivalent of a magnitude 5.4 earthquake. No one was injured, though kayakers on a nearby island lost their gear and a cruise ship in the neighboring arm felt strong currents and sudden water-level changes.
Before-and-After Views from Orbit
Landsat 8 and Landsat 9 captured clear images of the same stretch of fjord on July 26 and August 19. The earlier scene shows continuous green forest along both shores. The later image reveals a bright scar on the north wall where the landslide originated and a wide brown band where the tsunami removed trees.
Sawyer Island, roughly six miles down the fjord, also shifted from green to brown. Only scattered trees remained at its highest elevations. The front of South Sawyer Glacier retreated noticeably in the same short interval, partly from natural melting and partly because the landslide sheared off a large section of the glacier’s terminus, sending icebergs into the water.
What Researchers Learned from Multiple Data Sources
Geomorphologist Dan Shugar of the University of Calgary and his team combined satellite imagery, airborne surveys, ground observations, eyewitness accounts, and computer simulations to reconstruct the sequence. Their full analysis appeared in the journal Science on May 6, 2026. The work confirmed both the height of the tsunami run-up and the prolonged seiche that followed.
Exactly why the slope failed remains unclear. Moderate rainfall before the event and the glacier’s rapid retreat both likely played roles by removing support from the valley wall. The retreat also exposed new open water, allowing the tsunami to propagate farther than it might have otherwise. Shugar noted that renewed glacier retreat visible in satellite data can serve as an early warning that slopes above the water have become more unstable.
Further monitoring will help determine whether similar events become more frequent as glaciers continue to shrink in the region.
Looking Ahead for Tracy Arm and Nearby Waters
The fjord remains connected to Stephens Passage, a busy waterway used by cruise ships and smaller vessels. Some operators have already adjusted routes to avoid the area while conditions stabilize. The stripped shorelines and floating ice will take years to recover, and the altered landscape may influence local wildlife and sediment patterns for some time.
Scientists continue to watch the site with both satellites and field instruments. The Tracy Arm event adds to a growing record of landslide-generated tsunamis in glaciated fjords, offering a clearer picture of how quickly these systems can reorganize when slopes lose their glacial support.