Deciphering the Origin of Waterfalls from Morphology in the San Gabriel Mountains, California

Abstract:

River channels record changes in climate and tectonics by adjusting their slope and morphology. Locally steepened sections of rivers, termed knickzones, have been used to indicate past external forcing; however, emerging research suggests that the waterfalls frequently found in knickzones can form autogenically as bedrock bedforms in the absence of external perturbations. Understanding the mechanisms that form these autogenic waterfalls and their resulting morphology is critical to develop accurate interpretations of geologic history from river longitudinal profile analysis. Here, we use new measurements of the frequency of waterfall occurrence, waterfall drop height, and spacing between waterfalls to test whether waterfalls occur with a characteristic frequency, consistent with the formation of bedrock bedforms.

Results from analysis of over 100 waterfalls in the San Gabriel Mountains, California, shows that as riverbed slope increases, waterfalls occur with a higher frequency and tend to have a higher ratio of waterfall drop height to spacing between waterfalls. These findings are consistent with cyclic step theory, suggesting that waterfalls may form autogenically as bedrock bedforms with a characteristic morphology under set river conditions. Measurements such as these may provide a way to distinguish externally forced from autogenic waterfalls in nature. By determining whether the waterfalls that comprise regional scale knickzones are autogenic or externally forced, we can gain a better understanding of how to interpret past geologic events from topography.