TRACKING PAINTED PEBBLES IN THE MOJAVE- OFFROAD VEHICLES AND THEIR IMPACT ON SEDIMENT TRANSPORT

Since February 2000, we have tracked 1600 painted pebbles across 4 sites in the Mojave Desert, Southern California: the Iron Mountains, the Chemeheuvi Mountains, the Goldstone Deep Space Communications Complex, and Ft. Irwin. Three sites are in areas not disturbed by humans; the fourth, located in Ft. Irwin, is on a surface that experiences extensive off-road vehicular use. We find that off-road vehicle use dramatically accelerates pebble movement.

At each site, 400 numbered pebbles, 1-cm diameter, were laid out along orthogonal 20-m lines and surveyed (total station, 1-cm accuracy) 2-4 times a year to quantify pebble displacement and determine how vehicular traffic affects pebble movement. Pebbles are moved short distances by small rainstorms and animal activity. Pebble tracking demonstrates that animal burrowing, as well as tracked and wheeled vehicles move sediment up as well as down gradient. Large rainstorms, generating channelized flow, and vehicles disturbing the desert surface, move pebbles greater distances.

The average pebble movement in the 2 natural systems is between 4 and 6.5 cm/year but as high as 18 cm/year at the Chemeheuvi site, where runoff in one channel transported pebbles up to 31.5 meters in at least two transport events. These pebbles were not buried in the bedload and are still easily found downstream on the channel surface. All data are right skewed so median speeds for natural systems (2.9, 2.8, and 2.7 cm/year) are less than average speeds and cluster closer together because the outliers moved in channels inflate average velocities. In contrast, the average and median pebble speeds at the disturbed Ft. Irwin site are both 30 cm/year. Average pebble movement at Ft. Irwin is not levered by a small number of pebbles that have moved great distances, rather a large number of pebbles have been moved by vehicular disturbance.

The surface disturbance caused by vehicles is very different than the natural disturbance regime. In active training areas, pebbles get caught in tank tracks and tires and are either buried or scattered. Recovery of pebbles and average pebble movement are affected by the presence of offroad vehicles. The average recovery rate, after one year, of pebbles in natural systems is 98% but only 76% at Fort Irwin where vehicles repeatedly disturb the surface.