KNICKPOINT MIGRATION AND LANDSCAPE EVOLUTION ON THE ROAN PLATEAU

Evolution of a plateau landscape is paced by the rate at which rivers incise into the plateau margin, yet we know little about the specific processes involved, or what sets their rate. Sometime in the late Cenozoic, the upper Colorado River began to incise the landscape to the immediate south of the present Roan Plateau, in western Colorado. This led to elevational isolation of the Plateau, and to a wave of incision into its southern edge that constitutes a natural experiment we exploit. The incision resulted in the upstream propagation of roughly 30 knickpoints (over-steepened reaches in longitudinal profiles) on the Plateau, creating 60-meter waterfalls in some cases. That this natural experiment is performed in essentially uniform, flat-lying bedrock and in an area with relatively uniform climate implies that it should serve as an exacting test of our knowledge of the relevant bedrock incision processes.

As a first exercise, we use a stream power model to predict the locations of the knickpoints. This model reduces to one in which the recession rate of the waterfall is proportional to water discharge, and to rock susceptibility to erosion. We test variants in which a threshold power, and hence discharge, is allowed. We impose rapid incision of the Colorado River at a specified time to initiate knickpoint propagation. Models of the Parachute drainage (11 knickpoints) show the expected rapid initial propagation, which declines as tributary junctions are passed. The present locations of knickpoints can be reasonably predicted with a single rock susceptibility, with minimal erosional threshold. Tradeoff between rock susceptibility and timing of Colorado River incision makes the susceptibility constant ambiguous. To constrain independently the timing of initiation of incision, we are attempting to use the cosmogenic burial method to date a Colorado River terrace gravel deposit 200 meters above the Colorado River on Battlement Mesa, just south of the Plateau.