TEMPORAL CHANGES IN EFFECTIVENESS OF STORM FLOWS IN SOUTHERN CALIFORNIA CHAPARRAL FLUVIAL SYSTEMS AFTER WILDFIRE

Wildfire is a frequent disturbance to fluvial systems in semi-arid Mediterranean climates. During periods between wildfires, dense chaparral vegetation stabilizes weathered sediment on hillslopes. In steep basins, stream channels often contain step-pool morphology, with relatively stable boulder steps and cobble/gravel substrate between steps. Wildfire burns chaparral vegetation and releases a pulse of predominantly gravel-sized sediment that moves down hillslopes via dry-ravel processes. The first winter storm flows easily mobilize this fine sediment accumulated at the hillslope-channel margins, unlike the less mobile coarser material present before the fire. The transport and fluvial deposition of this fine sediment modifies channel morphology. We investigated geomorphic responses to a small storm (27 Feb – 2 Mar, 2014; ~40% of normal rainfall) in three fluvial systems within the Transverse Ranges in southern California with similar size, structure, and lithology. The basins burned at different times over the past several decades: a tributary to Matilija Creek (last burned in 1985); a tributary to Malibu Creek (last burned in 2007), and Big Sycamore Canyon (burned in 2013). The storm generated stream flow that was ineffective in transporting sediment or causing geomorphic change in the Matilija and Malibu basins, burned 29 and 7 years prior to the 2014 storm, respectively. In contrast, the fluvial flow in Big Sycamore Canyon, burned 9 months prior to the storm, substantially altered channel morphology by depositing relatively fine grained sediment (~0.41m deep along the thalweg profile) and by nearly burying step-pool structure. Results suggest that post-wildfire geomorphic processes generate transient sediment deposition that modifies the threshold of entrainment of channel bed sediment. Therefore, changes in geomorphic processes and sediment dynamics induced by wildfire vary over time and accordingly cause temporal changes in the effectiveness of storm flows.