STREAM SPEED BUMPS: HOW COMPETENT BEDROCK UNITS STALL AND MULTIPLY TRANSIENT KNICKPOINTS

Fluvial knickpoints are increasingly used to identify and constrain transient erosion, such as that following a change in rock uplift, base level fall, or climate change. Numerical models show that in the simplest setting (uniform geology), a single change in rock uplift rate, for example, is expected to generate a single wave of knickpoints, one in each tributary, that propagate up the fluvial network(s) at a uniform vertical rate. Following this model, every knickpoint in a single profile records a change in the stream network’s boundary conditions. How spatial variability in the bedrock geology may affect this seemingly straightforward signal has garnered less attention. Here, we demonstrate with a numerical stream model and field observations from the Arkansas River, Colorado, how erosion-resistant features with finite vertical dimensions, serve as speed bumps in the fluvial system. As expected, model results indicate that a propagating knickpoint stalls when a competent bedrock unit is encountered. As the knickpoint passes through this speed bump, two knickpoints emerge for a single perturbation of the boundary condition. The two emergent knickpoints upstream of the speed bump are the normal (in terms of propagation speed and height) knickpoint and a slower (stalled), lower knickpoint. We compare these results to stream profiles on the Arkansas River, Colorado, which record base-level fall since ~10 Ma and the consumption of the post-Laramide Rocky Mountain erosion surface. Profiles of tributaries upstream of the Royal Gorge generally exhibit multiple knickpoints. In contrast, tributaries that enter the Arkansas River below Royal Gorge are characterized by only a single prominent knickpoint corresponding to the edge of South Park. We suggest that the Arkansas River has experienced only a single major pulse of increased erosion, and that the multiple lower knickpoints observed above Royal Gorge are the consequence of the knickpoint stalling and splitting on lithologic speed bumps. These results show that transient knickpoints may arise from contrasts in bedrock erodibility, such as contacts; that knickpoints arising from lithologic contrasts need not be static features and need not be associated with contacts in the field.