Tracking painted pebbles: Short-term rates of sediment movement on four Mojave Desert piedmont surfaces

摘要

To quantify short-term sediment movement rates across Mojave Desert piedmonts, 1600 painted and numbered pebbles were laid out in paired, orthogonal, 20 m lines at 4 sites and resurveyed five times over 2 years and revisited 2 years later. Pebble lines cross shallow (5-15 em), ephemeral channels and adjacent unconsolidated interfluves, the latter being the dominant landform at all sites. Two sites are located on surfaces that have been or are impacted by military training activities, including the use of tracked vehicles. The two other sites have not been disturbed by human impact. Three different processes transport pebbles. Episodic streamflow in ephemeral channels transports a few pebbles long distances (decimeters to meters) down gradient. Bioturbation moves many pebbles small distances (centimeters) in any direction, and vehicular disturbance transports pebbles varying distances (centimeters to meters) in any direction. Significant down-gradient sediment movement occurred dominantly in channels where flowing water was concentrated. Interfluves were stable surfaces where little transport occurred. Off-road vehicle use is coincident with accelerated pebble movement. Pebbles moved further and faster down gradient at the disturbed Iron Mountain and East Range Road sites (mean speeds of 0.18 and 0.34 m yr~(-1), respectively) than at the undisturbed Chemehuevi and Goldstone sites, (mean speeds of 0.17 and 0.02 m yr~(-1), respectively). Mean pebble movement is highly and negatively correlated with vegetation density. Short-term pebble movement rates are several times lower than long-term (10~3 to 10~4 year) rates, suggesting the importance of rare, extreme precipitation events for sediment transport such as those of fall and winter 2004.