Receding Grewingk Glacier exposing rock fall, tsunami dangers

May 8, 2018

Grewingk Lake and Glacier.
Credit Bjørn Olson

  Across Kachemak Bay from Homer, just up the coast from Halibut Cove, is Grewingk Lake, a popular daytrip destination for visitors. Just a short water taxi ride from Homer, the lake is fed by its namesake glacier.

And like many glaciers, Grewingk is retreating because of climate change, exposing more mountainside to the elements and increasing the size of the lake. The combination of which could become problematic. That's because a rockslide splashing into the lake would likely create a destructive tsunami, according to former Kenai National Wildlife Refuge Ecologist Ed Berg.

It's happened before.

"In 1967 there was a major avalanche into the lake. Basically the face of the mountain ridge collapsed and slid into the lake. Partially onto the toe of the glacier," said Berg, currently a geology professor at the Kachemak Bay Campus of Kenai Peninsula College.

"But it created a terrific water wave that ran 200 feet up the hillside that scalped off all the spruce trees. And then this wave rolled down the outwash plain and carried these spruce trees into Kachemak Bay. And then it rolled into Humpy Creek which is on that same outwash plain, and wiped out a year's worth of salmon."

It was a rainy October day and no day-hikers were present, but the avalanche and tsunami would not have been survivable for anyone on the lake or at its edge. Fortunately, there wasn't much more in the way of damage at the time, but things have changed in the past 50 years.

"Well, the lake has pulled back maybe a mile-and-a-half since 1967. And it's exposed a much steeper and higher ridge, which basically it goes up to about 3,000 feet. And the angle is 45-degrees in some places," he said. "Well, we were curious to know how deep this lake and could another, bigger, wave be created."

What Berg found surprised him. Expecting a water depth of a couple hundred feet maybe, at the toe of the glacier the water was nearly 500 feet deep. What's worse, it sits below a 3,000-foot sheer rock wall formerly buttressed by the receding glacier.

"It's really a dangerous set up for another landslide," Berg said. "The rock is highly fractured. You can see faults running all over the place. Let's say the top of the ridge was to fall from 3,000 feet into 500 feet of water, you can imagine the size of the wave it will create."

Berg and his fellow researchers don't have to imagine -- he says they have computer modelers working on the calculations. But, he says the damage from a much larger tsunami than in 1967 would be more widespread than the loss of a hillside of trees and some salmon.

"Potentially the wave could be big enough that it could impact the Homer Harbor. So the question is, can you predict, or forecast, this kind of thing. There are many places, especially in Europe, who've been studying this. We're hoping to get some instruments in place to monitor the creep of the rocks, in essence. And also using some satellite imagery and set up some sort of warning that could be generated."

Berg said about 20-to-30 people visit Grewingk Lake daily in the summer, which is a short, eight-mile boat ride from the Homer Harbor.