The Recurrence of Large Boulder Movement in Small Watersheds of the Anza Borrego Desert, California

Abstract

Once in the channel of a small drainage basin, large boulders may remain stationary for long periods of time and become more or less permanent features of the channel’s geometry. A large boulder’s stability results from its high mass and consequent high tractive competence, coupled with the region’s sparse rainfall and the typically low stream flow regime associated with the small catchment area. Boulders in three small drainage basins in the Anza Borrego Desert, California were studied. The critical velocity of water flow required to initiate boulder movement on the channel floor, and the associated competent stream discharge and precipitation intensity required to produce this threshold, were computed. This study shows that rainfall intensities required to produce a flow of water sufficient to move large boulders in small drainage basins must approach or in some cases exceed the probable maximum precipitation. Given that the required precipitation intensities needed to produce stream flows capable of moving larger boulders in small desert drainage basins is extremely infrequent, these channel boulders must therefore be stable and persistent features in the present arid environment.