EOLIAN SAND SUPPLY TO COACHELLA VALLEY FRINGE-TOED LIZARD HABITAT IN THE NORTHERN COACHELLA VALLEY, CALIFORNIA

The Coachella Valley fringe-toed lizard (Uma inornata) is a federally listed threatened species that inhabits active sand dunes north of Palm Springs, California. Sand is delivered to these dunes by a combination fluvial and eolian processes. Fluvial sediment, 82% sand by weight, derives from headwaters in the San Bernardino and San Jacinto Mountains and is delivered to well-defined depositional areas on floodplains on the valley floor during floods that typically occur during El Niño years. Finer sediments are entrained in a largely unidirectional wind field created by westerly flow through San Gorgonio Pass and deposited downwind in unstable coppice dunes. These dunes are transitory and decrease in size as the supply of alluvial sand is depleted. 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 both the supply of fluvial sand and rates of eolian transport on an average annual basis. Using a model that combines regional flood-frequency relations with local sediment transport data, the depth of sediment supplied to the fluvial deposition zones is estimated at 20-50 mm annually. Sand-transport rates calculated from local wind speed and direction data deplete this sand supply in 1-2 years, a result consistent with the transitory nature of most of the dunes. Aerial photography indicates the persistence of eolian sand in parts of the depositional zones for longer periods, suggesting that our estimates of sand depletion may be minimums. The actual supply of fluvial sediment is occasionally greater than our average annual yields, and vegetation-created roughness may decrease sand-transport rates compared to our estimated rates. Eolian-sand depletion during drought periods indicates that fringe-toed lizards occasionally are stressed owing to natural habitat degradation.