INFLUENCE OF LARGE BOULDERS ON CHANNEL MORPHOLOGY AND SEDIMENT TRANSPORT IN BOULDER-BED STREAMS, NEAR SANTA BARBARA, CA

Steep boulder-bed streams found on the south flank of the Santa Ynez mountain range, near Santa Barbara, CA were historically productive regions for the endangered southern steelhead trout. These channels are relatively stable as they are dominated by large, debris-flow delivered boulders, often greater than 3 m in diameter, that are generally not transported by moderate stream flows. The boulders act as large roughness elements (LREs) that exert a major control on channel form and process and thus fish habitat, similar to large organic debris found in the heavily forested streams of northern California. Causative relationships between channel morphology and sediment transport can be established by determining the discharge and unit stream power responsible for the scour of pools and deposition of spawning gravels on riffles and gravel bars. We hypothesize that at a critical threshold value of unit stream power, pools are scoured as a result of channel constriction and convergence of flow between large boulders and stream banks or bedrock outcrops. With respect to deposition on riffles, boulder steps and areas of spawning gravels, we speculate that each class of gravels will be deposited at a threshold value of unit stream power. Additional sediment deposition and storage occurs behind key boulders, which trap smaller boulders in an interlocking stable framework. These keystone boulder frameworks often create steps, which store a range of mobile and immobile particle sizes and control local variance in unit stream power and potential energy. Preliminary results derived from the HEC-RAS v. 3.0 hydrologic modeling tool support our basic assumption that the formations of pools and riffles in boulder-bed streams are largely controlled by changes in unit stream power caused by the LREs. Variance in mean velocity and available shear stress also appears to be tightly coupled with the occurrence of these large, stable boulders.