Autocyclic Formation, Retreat, and Destruction of Waterfalls in an Experimental Bedrock Channel

Abstract:

Waterfalls are ubiquitous in steep landscapes and have been documented to retreat upstream at rates far outpacing standard fluvial incision into bedrock. While the formation of waterfalls following changes in climate and base-level lowering have been well-documented, little work has explored the formation of waterfalls via the internal dynamics from interacting flow hydraulics, sediment flux, and evolving channel morphology. Distinguishing between waterfalls formed via external versus internal forcing is important, as waterfall formation and retreat rate is often applied in inverse to determine the timing of external forcing. Here, we present results from a laboratory experiment designed to explore channel incision and waterfall formation. We fed water and sediment at constant rates over an initially planar surface at 19.5% slope. A channel rapidly incised into the artificial bedrock substrate, and small-wavelength variations in erosion rate created steps and pools which grew in amplitude. As pools deepened, erosion was focused on the upstream pool faces creating steep segments in the channel bed. At the topographic breaks between these steep segments and their upstream treads, water detached from the bed forming ventilated waterfall jets which impacted the plunge pools below. Continued pool deepening led to sediment deposition on the pool floors, locally inhibiting vertical incision while the upstream and downstream surfaces were free to erode. Amplified erosion at the waterfall lip incised a new pool into the bedrock previously composing the waterfall face while simultaneous lowering of the downstream pool lip resulted in the destruction of the original pool. Repetition of this process in our experiment suggests that interactions between bedrock erosion and sediment cover can result in the formation of a series of plunge pools which retreat upstream, and that care must be taken to distinguish between autocyclic versus allocyclic waterfall formation in studies examining landscape response to external forcing.