ABSTRACT Quantifying fluvial response to extreme floods is often limited by the absence of detailed pre‐event topographic and sedimentological data. This study takes advantage of a rare monitoring dataset in the Guarda Mor River experimental catchment, southern Brazil, where a detailed baseline characterization (2021–2022) preceded an exceptional flood in April 2024. The event generated an estimated peak unit stream power (ωp) of approximately 3400 W m −2 , far exceeding established thresholds for significant geomorphic work. To integrally assess the impacts, we replicated the pre‐event surveys, focusing on: (i) geomorphological changes, via DEM of Difference (DoD); (ii) sedimentological adjustments, via surface and subsurface particle size distribution (PSD) analysis; and (iii) alterations in hydraulic geometry and flow resistance. Results reveal a complete geomorphic reset of the channel. The DoD quantified substantial morphological reconfiguration, including an average 24% channel widening and a transition from upstream erosion (maximum −0.73 m) to downstream deposition (maximum 1.19 m). Sedimentologically, the energy of the event was sufficient to break the pre‐existing armour layer, reducing the armour ratio from 7.9 to 4.9 and inducing intense winnowing of subsurface fines (a nine‐fold increase in subsurface D50). This reveals a striking hydrogeomorphic duality: despite the significant coarsening of the surface layer (D50 increased from 192.5 to 249.6 mm), the structural reorganization led to hydraulic smoothing and increased efficiency. Flow resistance decreased as Manning n was reduced from ≈0.04–0.05 to ≈0.02–0.03, consistent with a reduction in topographic roughness ( σ z). This shift during high‐flow events results in higher flow velocities and a transition from a subcritical to a critical/supercritical flow regime (Fr 1). The event reset the channel to a new, more hydraulically efficient state; however, due to the armour disruption, it is now significantly more sensitive to subsequent flow events.
FEYH et al. (Fri,) studied this question.