South Sawyer Glacier is among the most visited glaciers by recreational boaters cruising beautiful Southeast Alaska. Small cruise ships out of Juneau also venture up Tracy Arm, an impressive 30-mile-long fjord with 4,000-foot-high granite walls, deep valleys, and waterfalls. Holkham Bay serves as the entrance to both Tracy Arm and Endicott Arm and their respective tidal glaciers. South Sawyer Glacier is perhaps the easiest glacier in Southeast Alaska for recreational boaters to reach due to fewer ice bergs in the passage. But a tsunami is not top of mind for most boaters traveling the fjords of the Inside Passage.
Tsunamis can be localized events (a seiche, a trapped tsunami) triggered by massive landslides. And that’s what happened on Sunday, August 10, near the head of Tracy Arm. We’re not talking a top layer of soil giving way, but rather a large mass of bedrock involving tens of millions of cubic meters of material sliding into the sea, which set off waves of 10-15 feet high in this case. Wave heights in some locations were reported to be higher due to amplification effects within the fjord. With intense rainfall events, pre-existing slope instability, and glacier retreat, favorable conditions are created for large mass movements.
The Alaska Earthquake Center, located at the University of Alaska Fairbanks Geophysical Institute, reported that the event on August 10 was equivalent to a magnitude 5 earthquake, and is one of the largest slides in at least 10 years. This landslide-created tsunami is considered a rare geologic event; hundreds of very small tremors were detected prior to the massive landslide, also something not often observed. The Alaska Earthquake Center is hopeful that a study of this event provides the opportunity to learn more and be better prepared.
Thankfully, the landslide and resulting seiche tsunami occurred at 5:30 am local time, when cruise ships and recreational boaters were not in the fjord during the massive fall of rock. No deaths or injuries occurred, however, there were boats and kayakers not far from the event. The small cruise ship, the David B, was anchored near the entrance of Fords Terror located along Tracy Arm, where they saw the tide quickly rise and fall by approximately 10 feet.
Of greatest safety concern was a group of kayakers, who were camped on Harbor Island in Holkham Bay near the entrance to Tracy Arm and Endicott Arm. A tidal surge swept away half their gear, including one kayak, their personal gear, and cooking equipment. The group was kayaking the Inside Passage from Washington State. The kayakers sent out a call for help at 11:30am on Sunday via their VHF radio, and were picked up by a charter yacht, the Blackwood. The vessel’s tender fought strong currents as the kayakers’ remaining belongings were loaded in the tender. Before returning to Juneau, the Blackwood motored up Tracy Arm to see the damage left from the tsunami, only one tree was still standing on Sawyer Island.
Header Photo: Brian DeNault
Waggoner Field Correspondent
Holkum Bay Photo: Lorena Landon
