PRESERVATION OF HOLOCENE WIND VECTORS BY VEGETATION AND VENTIFACTS IN AN EOLIAN SAND SYSTEM, NORTHERN COACHELLA VALLEY, CALIFORNIA, USA

Sand is delivered to active sand dunes north of Palm Springs, California by a combination of fluvial and eolian processes. Fluvial sediment from headwaters in the San Bernardino and San Jacinto Mountains is delivered to depositional areas on the valley floor. Sediment is then entrained in a unidirectional wind field created by westerly flow through San Gorgonio Pass and is deposited downwind in transitory coppice dunes that become depleted as the supply of alluvial sand is exhausted. Wind energy in the Coachella Valley is abundant, and eolian sand transport is limited solely by the supply of suitable fluvial sediment. A long-term sediment budget for the Coachella fluvial-eolian system is estimated by linking models of watershed sediment yield and eolian transport to quantify the supply of fluvial sand and eolian activity. Data on wind velocities and directions are sparse despite extensive wind-energy development. At Palm Springs Airport, south of the center of strongest winds, sand-transporting winds (>19 km/hr) have a mean vector of 298°, indicating a dominant west-northwesterly pattern. One year of wind data at Edom Hill yielded sand-transporting winds of >23 km/hr with a mean vector of 282º. Desert plants and ventifacts preserve prevailing wind directions for periods that extend through the Holocene. Long-lived creosote bush (Larrea tridentata) is commonly wind sculpted and forms linear clonal mounds up to 18 m long that are hundreds to thousands of years old. In the center of the wind field, typical vectors on Larrea are about 311º±18º. Ventifacts are formed on boulders lying on surfaces mapped as early Holocene in age. At the center of the wind field, wind directions indicated by ventifacts have an average direction of about 293º±17º. On the margins of the wind field, the vectors diverge and some ventifacts appear to have preserved two directions, possibly suggesting a shift from westerly to northwesterly flow. Short-lived shrubs also preserve wind vectors that may markedly diverge from the other evidence. Vector evidence also preserves the divergence of wind around large landforms. A comparison of short-term historical records and long-term bio-indicators suggests that mean wind direction in the Coachella Valley has shifted in the Holocene.