Trajectories of River-Floodplain Adjustments Following Compounding Wildfire-Flood Disturbances

Wide, low-gradient segments within river networks (e.g., “beads”) play a critical role in absorbing and morphologically adapting to disturbances, such as wildfires and debris flow. However, the magnitude and rate of active channel morphological adjustment compared to pre-disturbance conditions and the post-fire response of in-stream restoration features and geomorphic units is not clearly understood. To better understand the impact of major disturbances on river beads, we analyzed trajectories of river morphology adjustments following the 2020 Cameron Peak wildfire and 2022 flood and debris flow at Little Beaver Creek, Colorado, USA. We used historical National Agriculture Imagery Program imagery (2009-2019) and post-fire drone-imagery surveys (2021-2023) to assess morphological change in a 500-m, low gradient bead of Little Beaver Creek. We analyzed remotely sensed imagery for pre- and post-geomorphic metrics in rates of floodplain destruction and formation, and changes in channel width and channel migration. Rates of floodplain destruction and formation, along with centerline migration greatly increased after the first post-fire runoff season and recovered to the historical range of metrics three years after the fire. The large flood in 2022 increased the rate of channel width reduction with immediate infilling of side channels, followed by the infilling of pools, and growth in bars and islands. The ability of the active channel of Little Beaver Creek to quickly adjust to fire and flood disturbances demonstrates the importance of river beads for enhancing river-floodplain resilience to large disturbance events, especially compounding hazards such as fires and floods. Our research also can inform river management and restoration about the importance of heterogeneous and dynamic river-floodplain systems to support resilient watersheds.