Photo of Tracy Arm after megatsunami on 10 August 2025 (from Wikimedia)

9 August 2026

A year ago, on 10 August 2025, a mega tsunami flooded the Tracy Arm fjord south of Juneau, Alaska. It began at the foot of the South Sawyer Glacier.

embedded Google map of showing South Sawyer Glacier at the head of Tracy Arm. Note Juneau near top of the map at Sitka near bottom left.

The tsunami was truly “mega” with a wall of water 1,578 ft tall (481m). To put this in perspective, the wave would have over-topped Acadia National Park’s highest peak, Cadillac Mountain (1,530 feet) and was almost twice the height of the U.S. Steel Tower in Downtown Pittsburgh.

The tsunami was triggered not by an earthquake but by a landslide at the point where the South Sawyer Glacier has retreated.

Fortunately it happened so early in the morning that no one was there. At midday as many as 100 sightseeing boats would have been in the fjord. This simulation by Patrick Lynett shows what it would have looked like from a boat viewing the toe of the glacier.

simulation by Patrick Lynett on YouTube

Here’s a simulation of the tsunami from above.

Within days USGS scientists conducted aerial surveys of Tracy Arm to measure the tsunami and the force of the water. They were amazed at what they found.

Scientists have known for years that glacial retreat can trigger landslides.

As glaciers retreat with ever-increasing speed, ice that for millennia has held up potentially unstable mountainsides disappears, a process called debuttressing. Geologists debate specifics of the process, but they acknowledge it contributes to slides, some of which are big enough to block rivers or trigger deadly tsunamis in lakes or oceans.

In a fjord the glacier carves a steep U-shaped valley. As it does so it loosens rock and exerts pressure against the walls it is creating. When the glacier retreats, the pressure against the walls disappears. Loose material sags and becomes a landslide.

screenshot from Glacier Valley Formation animation from Wikimedia

Here’s the origin of the screenshot showing the life cycle of a glacier and the walls it created. The animation does not show the landslides.

Glacier Valley Formation, animation from Wikimedia

What’s disturbing is that the frequency of glacial-retreat tsunamis has increased dramatically in the past 15 years in Alaska. Where is it dangerous? Should sightseeing avoid certain glaciers? Which ones? And what about coastal towns within reach of the tsunamis?

As climate change heats up, glaciers are retreating fast. Alaska is the poster child for climate change.