Aeolian Destabilization Along the Mojave River, Mojave Desert, California: Linkages Among Fluvial, Groundwater, and Aeolian Systems

A notable aspect of ecosystem deterioration in arid regions is sand movement subsequent to the loss of plant cover. The Lower Mojave Valley in the Mojave Desert of southern California is undergoing rapid environmental change owing to groundwater drawdown, causing the death of phreatophytic vegetation and the reactivation of aeolian sand. Historical documentation shows that in the early twentieth century, sand derived from the open river channel accumulated to form large stabilized nebkhas, anchored by mesquite (Prosopis glandulosa), a plant that requires the water table to lie 9 m or less below the surface. The natural balance between winds, sand supply, and vegetation changed in the later twentieth century, as increasing demand for groundwater caused the water table to drop from a near-surface elevation to as much as 30 m below the surface. The mesquite that anchored the dunes declined in vigor or died. Riparian vegetation, including cottonwood and willow, was scoured from the channel during flood events, and did not recolonize as the low water table prevented the establishment of seedlings. Extensive new areas of the channel were laid open to wind erosion. Sand derived from the channel and cannibalized from the degrading nebkhas began to migrate downwind. Long sand stringers extended from west to east across the dunefield. In the 1990s, barchans developed atop the sand stringers, growing rapidly from incipient low-amplitude forms to well-developed barchans 60 m in width with sharply defined slipfaces 6 m in height. The dunefield has changed from a predominant mode of deposition to one of erosion and transportation, in which the water table and vegetation no long exert an influence. Dune encroachment from migrating sand streaks and dunes buried equipment, rendered buildings and homes unusable, blocked the entrances to property and destroyed pasture areas. Changes to the aeolian environment of the Lower Mojave Valley reflect complex interactions between the wind regime, river channel morphology and sediment, surface and subsurface hydrology, geology, vegetation, and human influence. This study demonstrates the sensitivity of aeolian systems to land-use changes, particularly in environments where wind is not a limiting factor, and illustrates the rapidity with which such changes can occur.