Berlin, Maureen MÌý1Ìý;ÌýAnderson, Robert SÌý2
1ÌýÀÖ²¥´«Ã½/INSTAAR
2ÌýÀÖ²¥´«Ã½/INSTAAR
The Roan Plateau in western Colorado constitutes a natural experiment for studying landscape response to a drop in base level. Late Cenozoic incision of the upper Colorado River led to elevational isolation of the Plateau and initiation of a wave of incision into its southern edge. Knickpoints (oversteepened reaches that contain waterfalls 60–110 meters in height) mark the upstream extent of this headward-propagating wave. We describe how the structure and lithology of the Roan Plateau may be important in controlling the long-term evolution of this landscape. That this incision has occurred in a laterally extensive, well-stratified, and essentially flat-lying bedrock and in an area with relatively uniform climate implies that it should serve as a good test of existing knickpoint propagation models.
We predict the locations of knickpoints by using a stream power-based celerity model, in which horizontal knickpoint recession rate is a power function of upstream drainage area and is proportional to rock susceptibility to erosion. We choose to model horizontal knickpoint velocities because we assume that vertical knickpoint velocities have not been uniform over the duration of landscape response; vertical trajectories of the knickpoints likely reflect the structure and vertical variations in resistance of the sedimentary bedrock. We specify the timing of knickpoint initiation based on available Colorado River incision data; constraining the amount of base level fall by projecting relict longitudinal profiles downstream is problematic, because the upper plateau profiles may reflect the dip of the bedrock. Models of the Parachute and Roan drainages (17 and 16 knickpoints, respectively) show expected rapid initial knickpoint propagation rates, which decline as tributary junctions are passed. Modeled positions of knickpoints match remarkably well with the observed knickpoint locations, using a single combination of model parameters. We find that knickpoint speeds are roughly proportional to the square root of upstream drainage area. We compare our model results with past studies and explore how longitudinal profile analysis may be used to derive independently the exponent on drainage area in the celerity model. Our results emphasize the dependence of knickpoint migration rate on drainage area, and lay the foundation for more process-specific work.