276-9 QGG Stanley A. Schumm Award: Influence of Large Wood on In-Channel Sediment Storage after Post-Fire Debris Flows in Basins of Varying Vegetation.

Session: Natural and Urban Channel Responses to Disturbance, Management, and Restoration

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Abstract:

The frequency and intensity of wildfires are increasing across the western United States, altering landscape dynamics, and elevating the risk of debris flows in steep, fire-prone catchments. Although previous studies have established the geomorphic impacts of post-fire debris flows (PFDFs), less is known about how vegetation type influences the availability of large wood (LW) (e.g., diameter > 10 cm) in these channels and, subsequently, sediment storage post-fire. This study investigates the role of vegetation type in shaping in-channel sediment storage through the formation of valley-spanning log jams (valley jams) across three study regions: the Columbia River Gorge (OR), Big Sur (CA), and the San Gabriel Mountains (CA).

To evaluate the relationship between vegetation type, LW, and sediment storage, we used lidar differencing to detect geomorphic change at three moderate-to-high severity burn sites across contrasting vegetation environments—forested and chaparral. We used DEMs of difference (DoDs) from pre- and post-fire datasets to quantify topographic change to assess sediment deposition and erosion. We focused on valley jam formation and wood jam breaches. We used vegetation differencing to estimate pre-fire canopy height and infer large wood recruitment potential. This remote sensing was augmented with fieldwork at the three sites to validate the presence and composition of valley jams.

We hypothesized that sites with greater pre-fire canopy height—characteristic of forested environments—would exhibit greater in-channel sediment storage due to more abundant LW inputs and increased valley jam formation. Lidar differencing findings reveal abundant topographic signatures of valley jams in forested and mixed-vegetation catchments, but sparse or none in the chaparral-dominated catchments. Field observations of LW density are consistent with our hypothesis that valley jams more frequently form in more forested settings. Though LW density varied systematically with upstream contributing basin area and forest cover type, basin forest cover is the primary driver of LW density in channels. For example, in smaller drainage area catchments, LW density decreased systematically with basin-wide forest cover.

These preliminary findings suggest that vegetation type impacts post-fire sediment deposition dynamics through its influence on LW recruitment, with implications for debris flow magnitude, sediment delivery, and downstream hazard potential. Future work will quantify the storage volumes associated with observed valley jams to allow for a quantitative comparison between LW and sediment in variously vegetated environments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6587

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