Linking fan surface morphology to erosional process regimes: A morphometric framework

Fans formed by the accumulation of debris flows, rockfalls, and fluvial action are often found at the base of steep escarpments, which commonly border infrastructure corridors. Escarpments evolve at discordant rates, due to climate and tectonic gradients, and geologic heterogeneity. The variations influence watershed morphology, which in turn determines the dominant erosional processes and potential hazard along fans, such as rockfall, landsliding, avalanches, or flooding. Classically, erosional process regimes have been determined from simple morphometric indices, such as watershed length and the Melton Ratio (watershed relief divided by the square root of area; Wilford et al., 2004). Here, we propose a new framework that leverages high-resolution topography and its topographic derivatives along the steep Columbia River Gorge (CRG) escarpment (Oregon, USA) to classify erosional process regimes across 78 fans. Using fan slope, surface roughness, and drainage density, we map transitions from colluvial to debris-flow dominated fans. We show these topographic derivatives along fans can stand alone, providing the ability to distinguish upslope catchment processes without catchment morphology. This work serves as a framework for preliminary assessment of regional-scale process variability on fans, with applications ranging from hazard mitigation efforts to planetary geomorphology.