THE REMARKABLE TAHOE TSUNAMI STORY, TAHOE'S GREATEST CATASTROPHIC EVENT

Lake Tahoe is one of few lakes around the world known to have experienced a dramatic history of faulting, earthquakes, landslides, and, especially, tsunamis (Dartnell, 2018; Gardner et al., 1998, 1999, 2000; Howle et al., 2012; Kortemeier, et al., 2018; Moore et al., 2006, 2014; Schweickert et al., 2000, 2004, 2009, 2011; Schweickert, 2024; Ward, 2010, 2013). New field and underwater observations in many areas are presented here.

Previous work reveals that sometime between about 22,000-12,000 years ago, a ~12.5 km³ collapse of a large part of the western shore area of the lake between two active normal faults, produced a 14 x 12 km debris avalanche that spread across and covered the central third of the lake floor. The collapse and debris avalanche generated immense tsunamis with waves possibly up to 50-100 m (160-320 ft) in height, which swept across many areas aound the lake. In several shallow-water areas, tsunami waves produced sets of giant megaripples (or boulder ridges). Up to 1/3 of the water volume of the lake was emplaced temporarily on land by these waves and then rapidly flowed back into the lake. At least 15 drainback channel systems (some up to 4 km wide) with large-scale, deep (up to 160 m deep), drainback canyons were carved into the steep sidewalls. Extensive debris and sediment were carried down the drainback channels onto the flat lake bottom at 500-m-depths and large fields of sediment waves were produced.

New work indicates that much of the low-lying South Lake Tahoe area (with up to 9 km runup) was inundated and parts of glacial moraines in several areas were damaged or swept away by the tsunamis. Giant megaripples (boulder ridges) also developed in the Emerald Bay and Fallen Leaf Lake areas. Many other areas around the lake which show evidence of tsunami erosion and examples of deep drainback canyons carved in proto-Tahoe sediments will be discussed.

Many of these features have never been documented in any other lake. Lake Tahoe's history has revealed features to be sought around other major lakes in tectonically active regions.

I am indebted to collaborators J.G. Moore, C.A. Kitts, M.M. Lahren, J.F. Howle, and W. Kortemeier.