EFFECT OF CHAPARRAL WILDFIRE ON DRY RAVEL PROCESSES

Dry ravel, the sliding of sediment under gravity, transfers material from hillslopes to channels. Following wildfire, dry ravel greatly increases in the absence of vegetation on hillslopes, thereby contributing to the sediment supply at the landscape scale. Dry ravel has been documented following fires in chaparral environments in southern California. Although dry ravel may be a characteristic post-fire response in chaparral ecosystems, the process is not well documented, nor is its effect well understood, making prediction of post-fire flood hazards challenging. We address post-fire changes in steep channels with step-pool morphology in two basins in southern California’s Transverse Ranges where dry ravel is a dominant process. Big Sycamore Canyon was entirely burned during the May 2013 Spring Fire. Following the fire, ash and a charred layer covered hillslopes and ephemeral stream channels. We distinguish post-fire dry-ravel deposits as gravel-sized sediment overlying the charred surface layer. During the dry summer months following the Spring Fire, active dry-ravel processes produced numerous deposits that merge at the base of hillslopes at the margins of stream channels. Such deposits comprise a significant post-fire sediment source comparable to the dry ravel that we documented previously in the Transverse Ranges. After the Wheeler Fire (1985), detailed measurements of dry ravel taken in a tributary to the Matilija watershed indicate sediment contributions of 0.20 m³ per meter length of channel per month. In both study areas of Big Sycamore Canyon and Matilija Creek (drainage areas ~2.0 km²), large contributions of post-fire dry ravel are significant for the following reasons: the smaller size of sediment entering channels decreases the critical shear stress needed to transport sediment; the potential to transform the hydraulic and ecological functioning of steep channels when pools fill with fine gravel; and sedimentation hazards to downstream areas. The mobilization and transport of sediment contributed by dry ravel processes may be episodic and transient, modulated by the magnitude, frequency and duration of post-fire storms. Nevertheless, their effect on step-pool systems in chaparral environments may be growing in an era of more frequent and larger wildfires as a result of global warming.