EFFECT OF POST-WILDFIRE SEDIMENTATION ON PHYSICAL HABITAT FOLLOWING HIGH INTENSITY RAINFALL

Steep fluvial ecosystems in the southern California Transverse Ranges are characterized by step-pool morphology, coarse substrate, relatively narrow channel widths, and prominent hillslope-fluvial interactions. Instream biota are adapted to episodic changes in streamflow in this semi-arid environment. Recuring wildfires denude chaparral vegetation and cause physical changes to upstream and upslope sediment sources. A dry-season sediment pulse follows and continues via various geomorphic processes through the first (or more) storm seasons. In turn, this post-wildfire sediment supply often results in channel sedimentation that dominates fluvial ecology. Morphologic surveys and grain size analyses were collected three years before the 2017 Thomas Fire in a second-order step-pool stream channel in the Matilija Creek system. The same channel was investigated again after a high intensity rainstorm shortly after the fire. As fire-related sediment was mobilized and transported, physical changes included deposition of ~1.0 m of sediment, on average. Subsequent flows incised ~0.3 m through this material during the first storm season, initiating recovery of the pre-fire bedform morphology that characterizes physical habitat. Comparison of post-wildfire changes after the 1985-Wheeler Fire burned the same watershed illustrates variability of channel responses to storms following wildfire. Results suggest that these responses are governed by highly localized variability in magnitude and intensity of rainfall during storms following the wildfires, posing challenges for post-wildfire management of hazards and ecology. Channel sedimentation may influence in-stream communities and physical habitats to varying degrees, depending on interactions between post-wildfire rainfall intensity, magnitude, duration, frequency, and timing of resulting stream flows. These factors influence the volume and size of sediment that must be transported for recovery of the physical conditions required for functional instream channel habitat.