Beyond the initial impact: Channel evolution following wildfire and debris flows in a mountain stream of the Upper Colorado River Basin

Wildfire-driven sediment pulses can radically alter river morphology, yet the spatial and temporal variability of channel response and recovery remain poorly constrained, especially in low-order headwater systems. This study builds upon prior work in a steep, fire-affected catchment in the Upper Colorado River Basin by documenting the geomorphic response to repeat widespread debris-flows during a high-intensity convective storm—three years after the wildfire and two years after initial debris-flow events. The storm remobilized sediment from previously activated hillslopes, delivering new debris-flow deposits to channel reaches that had been buried and partially reworked by intervening snowmelt events. We combine field-based mapping of repeat debris-flow deposits, in situ instrumentation data, and repeat cross-sectional surveys to characterize evolving channel morphology, and apply two-dimensional hydro-morphodynamic modeling to simulate sediment reworking and channel evolution across a range of flow scenarios. Cross-sectional analyses reveal continued channel adjustment consistent with a multi-year sediment-wave recovery trajectory punctuated by episodic sediment reloading. Together, field observations and modeling results indicate how debris-flow reinitiation can extend the geomorphic legacy of wildfire and provide empirical constraints for predicting river evolution in fire-prone landscapes, underscoring the importance of sustained monitoring and process-based modeling beyond the initial post-fire disturbance window.