Tsunami-generated boulder ridges in Lake Tahoe, California-Nevada

An array of east-trending ridges 1-2 m high and up to 2 kin long occurs on the Tahoe City shelf, a submerged wave-cut bench < 15 m deep in the northwest sector of the lake. The shelf is just north of the amphitheater of the giant subaqueous 10 km(3) McKinney Bay landslide, which originated on the west wall of Lake Tahoe. Images from a submersible camera show that the ridges are composed of loose piles of boulders and cobbles that lie directly on poorly consolidated, fine-bedded lake beds deposited in an ancestral Lake Tahoe. Dredge hauls from landslide distal blocks, as well as from the walls of the re-entrant of the landslide, recovered similar lake sediments. The McKinney Bay landslide generated strong currents, which rearranged previous glacial-derived debris into giant ripples creating the boulder ridges. The uncollapsed part of the sediment bench, including the Tahoe City shelf, poses a hazard because it may fail again, producing a landslide and damaging waves.