Knickpoint migration and landscape evolution on the Roan Plateau, western Colorado

How do landscapes respond to base level fall? To address this question, we study the Roan Plateau in western Colorado, which provides a natural experiment in bedrock river incision. Multiple knickpoints (&sim100-m waterfalls) constitute a headward-propagating incision wave presumably triggered by Colorado River incision to the south. We model knickpoint retreat with a stream power-based celerity model, in which retreat rate is a power function of drainage area and is proportional to rock susceptibility to erosion. Initiation of retreat at &sim8 Ma is constrained by Colorado River incision through the reconstructed confluence elevation of a resistant layer. A single parameter set predicts final knickpoint positions that agree well with observations. Simulations suggest rapid initial retreat rates (&sim7--12 mm/yr at the Colorado River confluence) rates decline with step-wise decreases in drainage area at tributary junctions to modern rates of &sim0.5--2 mm/yr.The knickpoints may separate an incised landscape below from a relict terrain above, which is unaware of base level fall. However, steepened channels upstream of knickpoints suggest that fluvial incision has reduced the relict portion of this landscape. Steepened reaches extend several kilometers upstream and slope is doubled relative to upstream concavity. This steepening may reflect the localization of waterfalls on a resistant sedimentary layer. Upstream waterfall retreat could lower the waterfall lip (if the layer dips upstream), causing base level fall for the upstream channel. Numerical modeling experiments suggest this mechanism is more consistent with our observations than erosion amplification due to flow acceleration at the waterfall lip.Downstream of knickpoints, canyon width mirrors canyon relief, suggesting cliff retreat depends on channel incision. Numerical modeling experiments indicate that channel incision within the canyon can be approximated by upstream translation of the modern longitudinal profile. The canyon planview shape may also translate upstream, with higher cliff retreat rates at the canyon head. Canyons widen by lengthening of a straight side-slope below the cliff that must efficiently respond to channel incision and transport material supplied from cliff retreat. Knickpoint retreat is the primary response to base level fall here, and produces steepened channels upstream and characteristic canyon morphology downstream.