Waterfalls Alter Reach-Scale Fluvial Erosion Rates: Evidence From Field Data and Process Modeling

Abstract

Waterfalls are often interpreted as transient, upstream-propagating features that mark changes in external conditions. Thus, waterfalls are commonly used to infer past tectonic and climatic forcing, making understanding the controls on waterfall erosion central to predicting how external perturbations move through landscapes. Surprisingly, there exist few direct field measurements of waterfall erosion, and existing waterfall retreat measurements are rarely paired with measurements of waterfall morphology and frequency, which, theory suggests, modulate retreat rates. This lack of data limits our ability to test existing theory and explore how waterfalls alter reach-scale bedrock erosion rates. Here, we use cosmogenic ¹⁰Be accumulated in bedrock riverbeds to measure erosion rates in fluvial reaches with varying waterfall frequency and morphology. We find that waterfall-rich reaches erode one to five times faster than the landscape average, and that reach-averaged erosion rates increase with increasing waterfall frequency. We develop a new, process-based model combining waterfall and planar-channel erosion to explore mechanistic controls on the relative erosion rate between waterfall-rich and waterfall-free reaches. This model predicts that reach-averaged erosion rates increase with waterfall frequency at low sediment supply, consistent with our field measurements, but that waterfalls can also slow reach-averaged erosion rates for high sediment supply, large grain sizes, low water discharge, or large plunge pools. Our work is consistent with previous suggestions that waterfall erosion rates may decrease in low drainage areas and can influence long-profile morphology.