Mono Lake, lying at the base of the east-facing escarpment of the Sierra Nevada in east-central California, is a highly saline remnant of a previously much larger and nearly 800-foot-deeper lake of late Pleistocene age. It has the following parameters: surface area, 77.2 square miles; volume, 0.901 cubic miles; mean depth, 61.6 feet; maximum depth, 169 feet; and surface elevation, 6392 feet as of July 1964.
Relatively steep (0.5°–5°) and irregular slopes form the southwestern, western, and northwestern floors of the lake. At depths greater than about 90 feet, however, the slope and relief of the bottom decrease markedly over most of the lake. This relatively flat region below a depth of 90 feet forms an arcuate depression, designated as Johnson Basin, that swings around the southern half of Paoha Island. Paoha Island is near the center of the lake and is composed of arched and intruded lacustrine sediments of probable late Pleistocene age. Submerged topography along the sides of this island is very irregular, and local relief of 20–30 feet is common. A generally linear depression, Putnam Basin, lies at the base of the eastern insular slope adjacent to cinder cones and associated andesitic flows on the island. Putnam depression contains the 169-foot deep of the lake. A relatively smooth bottom characterizes the portion of the lake lying east of Putnam Basin.
A prominent subbottom acoustic reflector underlies as much as 40–50 feet of water-rich acoustically translucent (at 38 kc/s) lake mud over the central area of Johnson Basin. The reflector ascends toward the sides of the lake and essentially crops out at a lake depth near 90 feet.
Most of the irregular relief of the lake floor, including the several islands in the lake, is thought to have been formed in Recent time. This relief has been produced by sublake faulting, volcanism, and gravity slumping of uplifted lacustrine sediments. Formation of the prominent subbottom reflector is attributed to the accumulation of water-rich highly flocculated sediments over more consolidated lake sediments of late Pleistocene age. Accumulation of the acoustically translucent lake mud probably began near the end of late Pleistocene time in response o t the formation of high salinity and to the cessation of rapid rates of Pleistocene lake sedimentation. Mono Lake has not desiccated in Recent time.