Big blocks and river incision: A numerical modeling perspective (talk at CU Hydrologic Sciences Symposium)

Bed sediment in bedrock rivers is often enriched in large (>1 m diameter) grains in knickpoints, or reaches where the channel is unusually steep. We hypothesize that clustering of large blocks in the Boulder Creek knickpoint and other steep reaches of mountain streams is a manifestation of an autogenic (internal) feedback generated by rapid river incision in well-jointed bedrock. Rivers, through erosion and steepening of their adjacent hillslopes, can force an increase in the delivery of large blocks to the channel that in turn shield the bed and retard river incision. Slowing of incision due to hillslope block delivery may inhibit knickpoint propagation and landscape adjustment. Here we use a numerical model to explore whether incision rate-dependent delivery of large blocks can explain the distribution of large grains in Boulder Creek, and whether this grain size distribution requires enough hillslope block delivery to alter channel form and adjustment rates. We argue that the channel-hillslope feedback implied by this grain size distribution may cause real landscapes to differ significantly from those predicted by present landscape evolution theory.

Model results show that when the adjacent hillslopes do not respond to rapid river incision (i.e., blocks are never supplied), the channel response to base level perturbations follows predicted shear stress behavior. When we account for block delivery from the hillslopes, our model successfully replicates the clusters of blocks in knickpoints noted in the field, suggesting that the feedback we describe may be responsible for the block size distribution in Boulder Creek and other natural settings. As block delivery increases, the reach profile form becomes more convex-upward and knickzones cease retreating up the reach at predicted speeds. Our results illustrate a major complication in using signals of landscape transience such as knickpoints to extract information about landscape history.