ALLUVIAL FAN RESPONSE TO CLIMATE CHANGE ALONG THE LOWER COLORADO RIVER

Alluvial fan aggradational phases along the lower Colorado River have been attributed to either (a) the wetter climates of the Pleistocene when stream power was greater, or (b) the warmer periods of the late Quaternary when lowered vegetation densities favored increased hillslope erosion. This study used optical dating methods to determine the timing of fan aggradation to better understand the potential climatic conditions responsible for aggradational events. Optical ages from fans along the Big Maria, Mohave, and Whipple Mountains suggest that the fills were deposited between ~ 35-10 ka, with most (7/9) indicating that fans aggraded during Oxygen Isotope Stage (OIS) 2. Records for fan aggradation during OIS 2 are relatively rare in the region, and six of these ages cluster between ~ 24-17 ka, a period of fan stability according to studies conducted in the nearby Mojave Desert. Including these ages with those from other Mojave and Sonoran Desert alluvial fan chronologies indicates that aggradation occurred under an apparently diverse set of climatic conditions, including the relatively cool and wet climates of the late Pleistocene, periods of warming associated with the Pleistocene-Holocene transition, and the warm climates of the Holocene. The similar responses of fans under each of these climatic conditions questions the link between fan aggradation and changes in temperature and vegetation cover that have been used in other studies. In order to explain fan activity throughout the late Quaternary, this study instead suggests that aggradational phases were a response to periods of greater humidity when precipitation levels were higher than normal. This argument is supported by comparing fan chronologies to paleoclimate records from the region. Periods of fan deposition throughout the late Pleistocene and Holocene correspond with pluvial phases of Lake Mojave and periods of regional dunefield stability, which are both local indicators for wetter conditions.