Owens Valley has supplied Los Angeles, California with fresh water since 1913 when surface waters were diverted through a 233-mile long aqueduct by the Los Angeles Department of Water and Power. Further diversion of Owens Valley surface water (including water from Mono Basin to the northwest) and installation of groundwater wells began in 1970. Development of this region as a fresh water source has altered surface and groundwater hydrology; Owens Lake, once 270 square km in area, is now completely desiccated. Previous hydrologic studies in Owens Valley have defined a water budget assuming steady state for modern climate. The major inflows to the lake were surface runoff and groundwater dominated by mountain block recharge (MBR). Since the valley is located within the Great Basin, it is hydraulically closed with surface evaporation as the only modern outflow; during the late Pleistocene, this lake overflowed to the south feeding a chain of lakes that had its terminus in Death Valley. In previous studies MBR was estimated based on measurement of playa discharge and an assumption of steady state. Recent results based on GIS analysis of modern climate have shown that modern MBR is inadequate to support observed groundwater discharge from the playa, thus leading us to question the long-held assumption of steady state. These results have led us to test the assumption of steady state in the undisturbed lake system. We use GIS and modern climate data to compare modeled and observed pre-diversion lake levels. We calculate lake depth, area, and volume from a lumped model of MBR, surface runoff, and lake evaporation. Management of the Owens Valley hydrologic system has always relied on an assumption of equilibrium between modern climate and surface and groundwater. If lake/playa hydrology is still influenced by late Pleistocene or early Holocene recharge, water managers may be overestimating available resources.