Waterfall formation and morphology governed by threshold channel slope: Field evidence from 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, we lack criteria to identify autogenic waterfalls, thereby limiting assessment of the ubiquity of self-formed waterfalls and their influence on past environmental forcing encoded in topography. Based on the assumption that autogenic waterfalls form from a specific type of bedrock bedform known as cyclic steps, we propose that autogenic waterfalls should form in series with waterfall height and spacing between waterfalls set primarily by channel slope. We use high resolution topography in the San Gabriel Mountains, California, to identify 360 waterfalls and show that waterfalls tend to form above a threshold channel slope of ~3-7%, coinciding with the onset of Froude supercritical flow, and that the waterfall height to spacing ratio increases with channel slope; consistent with cyclic step theory and previous laboratory experiments. Our results imply that in unglaciated mountain ranges with relatively uniform rock strength, the majority of waterfalls may be autogenic in origin, and show how threshold behavior can set basic channel patterns.