GEOMORPHIC RESPONSE TO SEASONALITY OF STORMS DURING THE HOLOCENE IN THE MOJAVE DESERT, CALIFORNIA

Ongoing research in Quaternary geomorphic evolution of the Mojave Desert is advancing understandings of geology-ecosystem links, climate history, and earth-surface processes. Luminescence and radiocarbon methods are used to date Holocene alluvial fan, lake, and river deposits, which improves knowledge of depositional timing and allows climate-landscape records to be evaluated. Data for ephemeral rivers and lakes indicate periods of sustained stream flow during the Little Ice Age (ca. 1300-1800 A.D.) and during a brief wet interval within the Medieval Warm Period (ca. 1050 A.D.). In addition, periods of alluvial fan aggradation are approximately 3-6 ka and 9-15 ka, and closely correspond to Gulf of California marine records of dominance by tropical diatoms and silicoflagellates. This suggests that periods of increased surface temperatures in the Gulf of California yield more frequent and more intense summer and fall monsoons, resulting in alluvial fan aggradation. In contrast, periods of enhanced winter Pacific frontal storms create river flow and ephemeral lakes. The Mojave Desert lies near both the Pacific coast and the Gulf of California, so experiences distinct storms from these sources. A partitioning of landscape-process responses results, which is correlated with seasonality of the dominant storms. Hillslope response to protracted low-intensity rain and snow, as opposed to intense rain, likely partitions the geomorphic response. This simple process-response model predicts that a future increase of sea surface temperature may cause enhanced alluvial fan sediment deposition, a process that many desert cities are ill-equipped to mitigate.