Trajectories of river-floodplain morphology and hydraulics following compounding wildfire-flood disturbances

Abstract

Wide, low-gradient segments within river networks (i.e., beads) play a critical role in absorbing and morphologically adapting to disturbances, including wildfires and debris flows. However, the magnitude and rate of morphological adjustment and subsequent hydraulic conditions provided by beads compared to pre-disturbance conditions are not well understood. This study analysed trajectories of river morphology, flood attenuation and hydraulic fish habitat following the 2020 Cameron Peak Fire and July 2022 debris flow and flood at Little Beaver Creek, Colorado, USA. Using repeat aerial imagery, ground-based surveys and hydrodynamic modelling, we assessed morphological changes in a 600-m-long bead of Little Beaver Creek. Metrics of floodplain destruction and formation and channel migration greatly increased in magnitude after the first post-fire runoff season but returned to the historical range of these metrics three years after the fire. The 2022 flood deposited sediment, infilled side channels, reduced pool area and increased the area of bars and islands. Flood wave attenuation and hydraulic habitat conditions did not show clear improvement or impairment despite more rapid changes in system geometry, geomorphic unit abundance and geomorphic unit location. The ability of the site to attenuate peak flows changed minimally and inconsistently over the studied floods. Various lotic habitat conditions changed—namely a reduction in floodplain access and deepening of certain pools—but the overall flow-type diversity of the system was not largely impacted. The resilience of the active channel of Little Beaver Creek to the fire and flood disturbances while retaining key services demonstrates the importance of river beads for enhancing river-floodplain resilience to large disturbance events and highlights river beads as key areas for preservation and restoration.