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Predicting sediment yield from recently burned areas remains a challenge but is important for hazard and resource management as wildfire impacts increase. Here we use lidar‐based monitoring of two fires in southern California, USA to study the movement of sediment during pre‐rainfall periods and postfire periods of flooding and debris flows over multiple storm events. Using a data‐driven approach, we examine the relative importance of terrain, vegetation, burn severity, and rainfall amounts through time on sediment yield. We show that incipient fire‐activated dry sediment loading and pre‐fire colluvium were rapidly flushed out by debris flows and floods but continued erosion occurred later in the season from soil erosion and, in ∼9% of catchments, from shallow landslides. Based on these observations, we develop random forest regression models to predict dry ravel and incipient runoff‐driven sediment yield applicable to small steep headwater catchments in southern California. Plain Language Summary: Wildfire makes watersheds more susceptible to hazardous flash flooding and debris flows, yet characterization and prediction of these hazards remains limited. In this study, we used repeat airborne laser mapping to quantify the movement of sediment in steep burn areas during initial dry periods and subsequent erosion from runoff events. Based on these observations, we developed two predictive models: one to predict the filling of channels with sediment prior to rainfall and a second one to predict erosion by debris flows and floods during initial storm events, which showed improvement over another commonly used model. After initial runoff events, much of the available sediment in channels was transported downstream, however small landslides and extensive erosion of soils across the landscape continued to supply sediment to floods and debris flows, in line with studies elsewhere showing continued debris flow activity despite reduced sediment in channels. Our study demonstrates that airborne laser mapping together with data‐driven modeling offer opportunities to increase predictive ability of post‐fire erosion and such approaches should be further explored in regions such as northern California, where fire is expanding and models of post‐fire erosion need to be tested and refined. Key Points: Postfire dry and wet sediment transport were quantified with lidar and dominant controls were identified using random forest regressionSlope and sediment supply, including dry ravel, were the strongest controls on initial sediment yield by debris flows and floodsContinued sediment bulking occurred from soil erosion and patchy mass wasting later in the wet season as channels became supply limited [ABSTRACT FROM AUTHOR]

Postfire debris flows are particularly complex to study because they do not form discrete initiation locations and commonly involve multiple simultaneously operating erosional processes. Although recent work has begun to elucidate a more mechanistic understanding of postfire debris flows, there is still a paucity of detailed sediment budgets characterizing these events. In this study, we seek to understand how postfire sediment sources and erosional processes change over multiple storm cycles. To do this, we performed repeat high‐resolution change detection in a headwater catchment burned by the 2018 Holy Fire in the Santa Ana Mountains, California, USA. This included terrestrial laser scanning in a zero‐order catchment (0.95 ha) and unmanned aerial vehicle structure from motion of a headwater channel network (up to 6.5 ha). During the initial storm events that produced runoff‐generated debris flows, we found that the evacuation of dry ravel and prefire colluvium accounted for half of the eroded material. These initial flows also acted to clear out much of the material stored within downstream headwater channel networks. In subsequent storm events of equal or greater rainfall intensity, total erosion from the study site was subdued, and the relative importance of shallow hillslope erosion from interrill and rill erosion was increased, as has been noted in similar studies in the region. Overall, this suggests that channel sediment supplies may be more rapidly depleted than hillslope sources, which may drive a trend of decreasing sediment fluxes over time from burned headwater catchments subject to repeated runoff events. Key Points: Repeat change detection was employed to explore the evolution of postfire sediment sources over a series of storms following wildfireHeadwater channel erosion decreased significantly over time, increasing the importance of shallow hillslope sources later in the seasonEvent‐scale sediment fluxes decreased over the season, which was driven by declining channel sediment supply [ABSTRACT FROM AUTHOR]