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Abstract Debris flows are powered by sediment supplied from steep hillslopes where soils are often patchy and interrupted by bare‐bedrock cliffs. The role of patchy soils and cliffs in supplying sediment to channels remains unclear, particularly surrounding wildfire disturbances that heighten debris‐flow hazards by increasing sediment supply to channels. Here, we examine how variation in soil cover on hillslopes affects sediment sizes in channels surrounding the 2020 El Dorado wildfire, which burned debris‐flow prone slopes in the San Bernardino Mountains, California. We focus on six headwater catchments (<0.1 km²) where hillslope sources ranged from a continuous soil mantle to 95% bare‐bedrock cliffs. At each site, we measured sediment grain size distributions at the same channel locations before and immediately following the wildfire. We compared results to a mixing model that accounts for three distinct hillslope sediment sources distinguished by local slope thresholds. We find that channel sediment in fully soil‐mantled catchments reflects hillslope soils (D₅₀ = 0.1–0.2 cm) both before and after the wildfire. In steeper catchments with cliffs, channel sediment is consistently coarse prior to fire (D₅₀ = 6–32 cm) and reflects bedrock fracture spacing, despite cliffs representing anywhere from 5% to 95% of the sediment source area. Following the fire, channel sediment size reduces most (5‐ to 20‐fold) in catchments where hillslope sources are predominantly soil covered but with patches of cliffs. The abrupt fining of channel sediment is thought to facilitate postfire debris‐flow initiation, and our results imply that this effect is greatest where bare‐bedrock cliffs are present but not dominant. A patchwork of bare‐bedrock cliffs is common in steeplands where hillslopes respond to channel incision by landsliding. We show how local slope thresholds applied to such terrain aid in estimating sediment supply conditions before two destructive debris flows that eventually nucleated in these study catchments in 2022. 

Abstract The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been limited due to expectations of: (a) decoupling between erosion rate and “threshold” hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published¹⁰Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in¹⁰Be erosion rates in catchments as small as 0.09 km², and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes. 

This dataset contains polygon shapefiles of watersheds draining detrital 10Be erosion rate samples from the San Gabriel Mountains, California (USA), with the naming format “mask_SampleID.shp”. This dataset is a companion to: DiBiase, R. A., Neely, A. B., Whipple, K. X, Heimsath, A. M., and Niemi, N. A. (2023), Hillslope morphology drives variability of detrital 10Be erosion rates in steep landscapes, Geophysical Research Letters, 50, e2023GL104392. https://doi.org/10.1029/2023GL104392 Full information for samples is described in: DiBiase, R. A., Neely, A. B., Whipple, K. X., Heimsath, A. M., Niemi, N. A., 2023. Compilation of detrital 10Be erosion rate data, San Gabriel Mountains, CA, USA, Version 1.0. Interdisciplinary Earth Data Alliance (IEDA). https://doi.org/10.26022/IEDA/112928. Accessed 2023-08-08. 

This dataset of detrital cosmogenic 10Be erosion rates from stream sands includes new and previously published measurements, compiled as part of DiBiase et al. (2023). Sample location information has been updated from original publications using field notes, pictures, and new lidar topography to align with correct stream network position. All erosion rates have been recalculated using updated in situ 10Be production rate estimates in quartz, as described in DiBiase et al. (2023). In addition to 10Be data, this dataset also includes catchment-scale topographic, climate, and landslide impact metrics, as described in DiBiase et al. (2023). 

Abstract Landscapes following wildfire commonly have significant increases in sediment yield and debris flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment yield is governed by processes that deliver sediment from hillslopes to channels, but it is commonly unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, which limits the ability to predict debris-flow occurrence and response to climate change. Here we use repeat airborne lidar topography to track sediment movement following the 2009 CE Station Fire in southern California, USA, and show that post-wildfire debris flows initiated in channels filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We found widespread patterns of 1–3 m of dry sediment loading in headwater channels immediately following wildfire and before rainfall, followed by sediment excavation during subsequent storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to differences in lithology, channel scour during storms did not occur. Our results support a fire-flood model in bedrock landscapes whereby debris-flow occurrence depends on dry sediment loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that sediment supply can limit debris-flow occurrence in bedrock landscapes with more-frequent fires.