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Abstract

Flood-induced geomorphic responses in post-glacial river systems are complex, where antecedent conditions strongly influence channel dynamics. This study examines the geomorphic effectiveness of historical large flood events and the exceptional atmospheric flooding in November 2021, specifically in the Coldwater River. We analyzed hydro-geomorphic indicators and their connectivity to assess the geomorphic effectiveness and 2021 flood exceptionality using hourly and daily discharge records, aerial photos, satellite imagery, and LiDAR data spanning the post-1950 period. Our findings indicate that the November 2021 flood exceeded the 100-year return period (RP) based on Log Pearson Type III flood frequency analysis, fundamentally resetting the geomorphic function of the river system. Geomorphic responses varied across reaches, especially the 100-year RP flood. Narrow, confined reaches with glaciofluvial terraces experienced extensive erosion and channel restructuring, while wider, unconfined floodplain-dominated reaches exhibited relatively minimal alteration. Historically, large floods (20-30 year RP) were incremental but significant adjustments in channel morphology, sediment redistribution, and planform evolution in the wider reaches, with minimal impact on narrow reaches. These channel responses were shaped by reach-specific factors, including valley confinement, sediment storage, stream power, and landscape characteristics, which collectively modulated the geomorphic effectiveness of large floods. Wider, sediment-rich reach continues to widen during both large and extreme floods due to its landscape characteristics as a sedimentation zone, where deposition processes and sediment storage remain dominant. In contrast, narrow, confined reaches experience pronounced erosion in glaciofluvial terraces only during extreme floods. The 2021 flood exceeded geomorphic thresholds across the basin, especially in confined reaches, leading to extensive terrace erosion and reshaping of channel-floodplain structures. The mean channel width underwent an exceptional expansion along the river. These findings highlight the interplay between flood magnitude and geomorphic conditions to landscape variability in determining flood-induced alterations. The exceptional geomorphic effectiveness of the 2021 atmospheric flood underscores the potential of extreme hydrological events to drive transformative landscape evolution. This study emphasizes the significance of integrating flood history and post-glacial landscape dynamics into fluvial systems and river restoration, contributing to improved flood risk management and advancing geomorphic research in dynamic landscapes.