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The Tahoe Tsunami

Once upon a time, geologists tell us, a massive chunk of Lake Tahoe’s western shore collapsed into the water in a tremendous landslide. The water responded by sloshing high onto the surrounding shores in a series of landslide tsunamis. A major new study in the journal Geosphere adds much new detail to that story, tracing massive features around and beneath the lake. And it places the date of the fearsome event near the time that humans first visited it.

Lake Tahoe is a peaceful mountain resort area today, but its geologic past has been long and violent. Its very presence is due to tectonic stretching of the Earth’s crust across Nevada, which has opened large basins from California’s Sierra Nevada crest all the way to Salt Lake City, Utah. The Tahoe basin has been there for roughly 3 million years, during which time it’s seen outbreaks of volcanism and countless major earthquakes.

Evidence of a catastrophe: Digital elevation model of central Lake Tahoe showing McKinney Bay (left) and landslide deposits crossing the entire lake. From USGS data

Forty years ago, the first sonar survey of Lake Tahoe showed evidence that bite-shaped McKinney Bay, in the middle of the lake’s western shore, is a scar left by a very large landslide and that huge pieces of that slide, as much as a kilometer long, are strewn across the lake bottom. Another sonar survey in 1998 revealed the slide in startling detail. Since that time, as funding permits, geologists have sent submarine-mounted cameras into the lake and taken core samples of its sediment. They’ve also tramped the shores and mountainsides, mapping the signs the catastrophe left there.

The new paper in the August issue of Geosphere, by veteran researchers James G. Moore, Richard Schweikert, and Chris Kitts, assembles the evidence old and new into a scenario of that convulsive day. Much of their paper represents significant progress on the Tahoe tsunami problem.

The landslide involved a body of rock made unstable by movement on a large-scale fault along the western shore. The slide, presumably triggered by an earthquake on that fault, sent some 12.5 cubic kilometers of rock and sediment into the lake, where it pushed a corresponding amount of water out of the way as huge tsunamis, perhaps 100 meters high. Much of this water burst over the lake’s outlet at Tahoe City and rushed down the Truckee River, where house-sized boulders litter the riverbed today as far downstream as Verdi at the Nevada border.

The rest of the water washed ashore all around the lake in what the authors call a “megasplash.” Not only was the water already very muddy from the landslide, but it washed away even more sediment, plucking the landscape down to bare rock wherever it went. The authors estimate that the lake was left perhaps 10 meters lower, because most of the lake is ringed with a terrace of fresh sediment at about that depth. The lake would have sloshed back and forth for days afterward, and surely more landslides were being triggered at the time. The sediment terraces probably built up over a period of weeks.

Ripples in still water: Giant ripple marks on the surface of the Glenbrook (top) and Logan Shoals sediment wave channel systems.From USGS data

These brand-new terraces, in turn, quickly began to collapse of their own weight, forming high-density “turbidity flows” that rolled down the slopes of the lakebed, digging channels along the way. The authors mapped 15 of these submerged channels around the lake that lead from the terraces down to the lake’s floor about 500 meters deep. At the bottom of the channels, the mudflows settled down into thick aprons of sediment, where they sit today. The channels and the aprons bear gigantic ripple marks that are as much as 400 meters from crest to crest. At left are two of those channel-and-apron complexes, directly across from the slide.

The authors say that the lake must have been muddy for years and its shores a barren wasteland. All of the mud gradually blanketed the whole lake bed, making the landslide-related features look much older than they really are. Mapping on land also delineated a sheet of clean sand as thick as 2 meters spread across the flatlands at Lake Tahoe’s south end, a sign that the area was swept by large waves. More detective work on land helped the authors narrow down the time of the megaslide and megasplash to some time between 21,000 and 12,000 years ago. All of this evidence fits into a terrifying picture of geologic uproar.

Such things are found elsewhere in the world, but not in a place as visible as Lake Tahoe. The authors conclude that “this one-time event in a closed lake basin provides a benchmark that can be compared to the processes that produced similar features in the ocean.” They also warn that the ground on either side of McKinney Bay is made of the same stuff and could cause similar events. These wouldn’t be half the size of the McKinney Bay slide, but they would be dire enough.

Andrew Alden is a KQED Science contributor.