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The evolution of a colluvial hollow to a fluvial channel with periodic steps following two transformational disturbances: A wildfire and a historic flood

Francis K. Rengers·Luke McGuire·Brian A. Ebel·G. E. Tucker
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Abstract

The transition of a colluvial hollow to a fluvial channel with discrete steps was observed after two landscape-scale disturbances. The first disturbance, a high-severity wildfire, changed the catchment hydrology to favor overland flow, which incised a colluvial hollow, creating a channel in the same location. This incised channel became armored with cobbles and boulders following repeated post-wildfire overland flow events. Three years after the fire, a record rainstorm produced regional flooding and generated sufficient fluvial erosion and sorting to produce a fluvial channel with periodically spaced steps. An analysis of the step spacing shows that after the flood, newly formed steps retained a similar spacing to the topographic roughness spacing in the original colluvial hollow (prior to channelization). This suggests that despite a distinct change in channel form roughness and bedform morphology, the endogenous roughness periodicity was conserved. Variations in sediment erodibility helped to create the emergent steps as the largest particles ( >D84) remained immobile, becoming step features, and downstream soil was easily winnowed away.

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Francis K. Rengers, Luke McGuire, Brian A. Ebel, G. E. Tucker. 2018. The evolution of a colluvial hollow to a fluvial channel with periodic steps following two transformational disturbances: A wildfire and a historic flood. https://doi.org/10.1016/j.geomorph.2018.01.003

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