302-3 Hazard cascades, mega logjams, and geomorphic response in mountain rivers

Abstract:

Hazard cascades, such as extreme rainfall-triggered landslides or wildfire-amplified debris flows, are increasingly recognized as major drivers of geomorphic change in mountain landscapes. Hurricane Helene illustrates this in the Southern Appalachians, where extreme precipitation in September 2024 triggered a compounding response of flooding, landslides and debris flows, bank erosion, floodplain deposition, and generational recruitment of large wood to channels. In several catchments, flooding and mass movements delivered pulses of wood that resulted in "mega logjams"—dense, 2-3 m tall, valley-spanning wood accumulations.  These mega logjams fundamentally altered sediment routing, hydraulics, and downstream response during and after flooding. These features sit at a critical but poorly constrained link in the hazard cascade connecting hillslopes to rivers: mega logjams may buffer hazard cascades by trapping sediment, additional wood, and flood energy, or become future hazards as wood decays or is remobilized during subsequent floods. These gaps in knowledge reflect both the transient nature of wood deposits and the rarity of both timely post-event data collection and long-term monitoring. This is especially pronounced in Appalachia, where a legacy of deforestation and channel wood removal has constrained observations and theory of wood-mediated disturbance dynamics.

Using pre- and post-event LiDAR, aerial imagery, and field surveys across six sites in the Blue Ridge Mountains of North Carolina, we document mega logjam morphology, spatial distribution, structural controls, and geomorphic impacts. Mega logjams formed in both wide, multi-thread reaches pinned by islands and in narrow valley pinch points. Preliminary results indicate these logjams initially act as major sediment and flood attenuators, dampening downstream geomorphic change. As we consider riverscapes in transition, understanding whether mega logjams function primarily as transient buffers, long-term sediment storage, or evolving hazards is critical for predicting the trajectory of hazard cascades and the resilience of forested mountain river corridors. Although motivated by Hurricane Helene, this research is broadly applicable to other disturbance-driven systems, including wildfire-affected watersheds where large pulses of wood may similarly reorganize river corridors.

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