Debris‐Flow Process Controls on Steepland Morphology in the San Gabriel Mountains, California.

Steep landscapes evolve largely by debris flows, in addition to fluvial and hillslope processes. Abundant field observations document that debris flows incise valley bottoms and transport substantial sediment volumes, yet their contributions to steepland morphology remain uncertain. This has, in turn, limited the development of debris‐flow incision rate formulations that produce morphology consistent with natural landscapes. In many landscapes, including the San Gabriel Mountains (SGM), California, steady‐state fluvial channel longitudinal profiles are concave‐up and exhibit a power‐law relationship between channel slope and drainage area. At low drainage areas, however, valley slopes become nearly constant. These topographic forms result in a characteristically curved slope‐area signature in log‐log space. Here, we use a one‐dimensional landform evolution model that incorporates debris‐flow erosion to reproduce the relationship between this curved slope‐area signature and erosion rate in the SGM. Topographic analysis indicates that the drainage area at which steepland valleys transition to fluvial channels correlates with measured erosion rates in the SGM, and our model results reproduce these relationships. Further, the model only produces realistic valley profiles when parameters that dictate the relationship between debris‐flow erosion, valley‐bottom slope, and debris‐flow depth are within a narrow range. This result helps place constraints on the mathematical form of a debris‐flow incision law. Finally, modeled fluvial incision outpaces debris‐flow erosion at drainage areas less than those at which valleys morphologically transition from near‐invariant slopes to concave profiles. This result emphasizes the critical role of debris‐flow incision for setting steepland form, even as fluvial incision becomes the dominant incisional process. Plain Language Summary: Steep landscapes evolve due to erosion by debris flows—sediment‐rich slurries that transport abundant wood and other debris—in addition to erosion by rivers and sediment transport on hillslopes. Although debris flows have long been recognized for their ability to erode channels and transport sediment, quantifying how they set the form of steeplands has remained a challenge. We use a landform evolution model that includes debris‐flow bedrock erosion to produce synthetic landscapes that exhibit similar patterns in valley‐bottom slope as those observed in the San Gabriel Mountains, California. Notably, we observe that the drainage area where simulated erosion by rivers begins to exceed that by debris flows does not correspond with the drainage area where the form of valley bottoms exhibits a transition. Rather, river incision outpaces incision by debris flows at drainage areas smaller than the morphologic transition point, implying that even as debris‐flow incision decreases in magnitude relative to river erosion, debris flows still have an outsized ability to dictate the morphology of steep landscapes. Key Points: Steepland morphology in the San Gabriel Mountains, California, corresponds with catchment‐averaged erosion ratesA landform evolution model with an incorporated debris‐flow process law reproduces relationships between morphology and erosion rateModeled fluvial erosion outpaces debris‐flow incision upstream from the transition between colluvial valleys and concave channels [ABSTRACT FROM AUTHOR]