Ground Truth Trekking

Grewingk Lake Tsunami Hazard

In 1967 the mountain above Grewingk Lake collapsed, generating a 60 m tsunami that flattened the forest in the Grewingk Valley. Since then, glacial retreat has caused the lake to expand and deepen, increasing the risk. Cracks in the ridge above the lake demonstrate the instability of the slope. Meanwhile, both the lake and ridge see far more visitors than in 1967 - there is an urgent need to understand the risk here.

Evidence of instability

The cracks and steps in this mountain ridge indicate that sometime in the past, the ridge shifted. There is some evidence this shifting motion is resuming, and might precipitate one or more large landslides. GET PHOTO

The cracks and steps in this mountain ridge indicate that sometime in the past, the ridge shifted. There is some evidence this shifting motion is resuming, and might precipitate one or more large landslides.

Advice for visitors

Grewingk Lake is an awesome place to visit, and well worth the trip. Doing so carries a small risk that you might be killed by a tsunami generated by a landslide in the lake. We are not recommending you stay away, but we do recommend a few steps that will help reduce that risk.

Don't camp at the lake. Camping at the lake greatly increases the time you're exposed to the landslide hazard, and if you're asleep you might miss warning signs that it's time to leave the lake.

Small landslides may be a warning sign. Take a look at this video of a landslide in the Phillipines - this is quite typical of rockslides: an extended period of small rockfalls preceding something much bigger. If you see rolling rocks, small dust clouds, or other signs of minor landslides, it might be a good time to leave the lakeshore and get to high ground on the Saddle Trail. Of course, such minor events may not be followed by a large landslide - small rockfalls are fairly common on the lake. But better safe than sorry. Also, we would very much appreciate reports of small slides - when they happen, and where they came from. Please email contact@groundtruthtrekking.org

Stay out of the Grewingk area after large earthquakes. Landslides sometimes follow earthquakes after some delay - sometimes even after days. This is particularly relevant after particularly large earthquakes with nearby sources.

If you witness a large slide, begin running immediately. If a substantial part of the mountain fails, likely producing a huge rumble and large cloud of dust, immediately flee the area near the lake - heading for high ground several hundred feet above the lake. Even if you see a wave that seems to dissipate after the initial splash, it may grow large as it approaches the beach. Often tsunamis can be escaped, if potential victims begin running immediately and don't stop until they reach high ground.

Map of Grewingk area

Crop of the summer 2018 version of GTT's Kachemak Bay State Park Map. GET PHOTO

Crop of the summer 2018 version of GTT's Kachemak Bay State Park Map.

Risk Factors

A number of risk factors point to Grewingk as a site that may experience a major landslide and tsunami in the coming decades. As yet, we are only at the initial steps of studying these factors to reduce risk to park users.

History of landsliding: Alpine Ridge above Grewingk Lake produced a huge (>80 million cubic meter) landslide that generated a tsunami which reached 60 m (200 ft) along the shore of the lake in 1967. Other areas further up along the ridge appear to be possible landslide scars from the more distant past. Landslides can recur in the same area - for example, Lituya Bay, famous for it's enormous landslide tsunami in 1958, has produced 5 such landslide tsunamis in the past few centuries.

Signs of weakness in the ridge above the lake: Alpine Ridge hosts a number of cracks, some near the edge, and others far into the ridge, which show it has weaknesses that are moving or have moved in the past. There is evidence of recent activity on some of these cracks, including active sink holes, stretched roots, and a drained lake.

Steep slopes above deep water: Grewingk Lake is in some places about 150 m deep (500 feet) and these deep areas sit below 1000 m (3300 ft) slopes that are steeper than the angle of repose. This arrangement creates the possibility for very fast-moving slides delivering a large portion of their energy to the lake, producing an energetic tsunami.