Morphologic signatures of autogenic waterfalls: A case study in the San Gabriel Mountains, California

Waterfalls can form due to external perturbation of river base level, lithologic heterogeneity, and internal feedbacks (i.e., autogenic dynamics). While waterfalls formed by lithologic heterogeneity and external perturbation are well documented, there is a lack of criteria with which to identify autogenic waterfalls, thereby limiting the ability to assess the influence of autogenic waterfalls on landscape evolution. We propose that autogenic waterfalls evolve from bedrock bedforms known as cyclic steps and therefore form as a series of steps with spacing and height set primarily by channel slope. We identified 360 waterfalls split between a transient and steady-state portion of the San Gabriel Mountains in California, USA. Our results show that while waterfalls have different spatial distributions in the transient and steady-state landscapes, waterfalls in both landscapes tend to form at slopes >3%, coinciding with the onset of Froude supercritical flow, and the waterfall height to spacing ratio in both landscapes increases with slope, consistent with cyclic step theory and flume experiments. We suggest that in unglaciated mountain ranges with relatively uniform rock strength, individual waterfalls are predominately autogenic in origin, while the spatial distribution of waterfalls may be set by external perturbations.