Late Quaternary Paleohydrology of the Eastern Mojave River Drainage, Southern California: Quantitative Assessment of the Late Quaternary Hydrologic Cycle in Large Arid Watersheds

The combination of historic climatic/hydrologic records and the latest Quaternary record from a climatically sensitive lake basin provides a unique opportunity for determining the variability of climatic regimes and hydrologic responses over the past 20,000 years within the Mojave River watershed, a large arid drainage basin in southern California. During the twentieth century, several short-term lakes formed in the playas at the terminus of the Mojave River in response to precipitation/flood runoff thresholds in the Transverse Ranges, 200 km away from the terminal lakes. Climatic forcing during lake-forming years, especially 1916, 1938, 1969, and 1978, approximate some of the climatic conditions between 22,000 and 8,700 years ago. Climatic conditions of historical lake-building flood events occurred when the position of the subtropical jet stream poured warm moisture-laden air mass, originating from the Pacific Ocean near the Hawaiian Islands, over the Transverse Ranges. Modeling results of these conditions suggests that (1) significant increases in storm frequency and related moisture in the Transverse Ranges, and (2) an order of magnitude increase in frequency of lake-building flood events along the Mojave River are required to maintain lake levels observed in the geologic record in the terminal basins.

Analyses of thirteen cores from pluvial Lake Mojave reveal prolonged latest Pleistocene and short-duration Holocene lake events in response to the increased frequency of large-volume flood events. Data from these cores reveal strong millennia climatic oscillations, expressed as high and low lake stands, which developed in response to large scale, oceanic-atmospheric phenomenon during the latest Quaternary. The last two climatic cycles represent the transition from Pleistocene to Holocene climatic conditions, lasting from 13,700 to 8,700 years ago, and appear to correlate with significant changes in vegetation throughout the Mojave Desert. Shorter-term cycles representing climatic oscillations over centuries occurred approximately 390 and 3,620 years ago, during known times of glacial advances throughout the world. Both of these short- and long-term cycles also appear to correlate to increased oceanic sedimentation rates, landslide failures, erosion and sediment yield, fill-terrace formation and alluvial-fan building within southern California. Future changes in climate and fluctuations in the hydrologic cycle that result in lake-building flood conditions could adversely impact human activity and man-made features within and outside the Mojave River watershed. Enhanced understanding of oceanic-atmospheric conditions producing these hydrologic events will enhance the future performance of the Mojave River hydrologic system.