HOW DO WATERFALLS INFLUENCE EROSION RATES?

Waterfalls are striking features that can self-form in steep streams and are a major component of fluvial erosion in mountainous rivers. Waterfalls can self-form at steep slopes as bedrock bedforms and erode via different processes that may result in faster or slower erosion than other parts of the river. Therefore, waterfall development may alter long profiles via creating autogenic knickpoints that have potential to obfuscate tectonic and climatic signals. Field studies have yielded insight into waterfall erosion rates; however, these studies typically deal with the retreat rates of tall (heights >5 times the flow depth) waterfalls that occur in isolation (i.e., they are not part of a chain of waterfalls occurring in series). It is unclear whether these results will hold for small (heights of ~1-2 times the flow depth) waterfalls that occur in series. We predict that short waterfalls erode differently as shallower plunge pools allow for more erosion over a greater distribution of discharges and lower jet angles allow for greater upstream propagation. Here we examine whether river reaches with small waterfalls erode faster or slower than reaches lacking waterfall using cosmogenic beryllium-10 (Be-10) erosion rates. We compare erosion rates in waterfall-dominated reaches with catchment-averaged erosion and find that all waterfalls erode faster than or equal to catchment-average rates. We furthermore analyze which factors drive erosion rates, including lithology, sediment, waterfall geometry. Preliminary analysis shows reach-scale erosion rates increase with increasing sediment flux, and increasing grain size, as well as with increasing waterfall frequency. We compare our field results with existing waterfall erosion models to examine how well they perform, and to calibrate a model that can interpret erosion rates from waterfall frequencies and channel morphology. This model will help interpret the impact of bedrock bedforms on reach-scale channel morphology and larger-scale river long profiles, thereby improving our ability to interpret tectonic and climatic history from river profile morphology.