Lake Tahoe was surveyed using a state-of-the-art, high-resolution, multibeam mapping system to provide an accurate base map for the myriad of ongoing environmental studies in and around the lake. The newly defined basin morphology shows steep basin margins on the northern, eastern, and western sides and a gentle margin on the southern side. Two large, flat plateaus several kilometers wide extend from the shore to about 40 m water depth in the northwestern sections of the basin. A series of ridges in the west and north are presumed traces of faults, some of which border the lake basin and some of which traverse across the northern section of the lake and converge in McKinney Bay. McKinney Bay is a large reentrant in the western margin that was created by a failure of the western margin that occurred about 300 ka. The failure generated a major debris avalanche that carried large blocks, some more than 1000 m long and 80 m high, accross the basin. Apparently, the debris avalanche was deflected by the eastern margin of the basin and flowed to the north and south. Small debris flows and slides have continued to occur in this area. Small debris aprons along the northern, western, and eastern margins, some apparently the remnants of collapsed terminal moraines formed in the basin from the 160 ka Tahoe Glaciation, which reached the edge of the basin. Eroded plateaus and ridges occur on a glacial outwash plain that covers the gentle southern margin. The plateaus and ridges are inferred to be remnants of another large terminal moraine of the Tahoe Glaciation.