The Influence of Hillslope Steepness on Sediment Supply Size Distribution along Rivers Draining the Colorado Front Range

Abstract:

The dynamics and timing of river incision are controlled by a multitude of factors including climate, topography, lithology, and sediment supply. Sediment size distribution affects fluvial erosion in rapidly incising rivers by setting the spatial frequency and temporal longevity of bed cover. Formulating accurate models of river erosion and landscape evolution requires constraints on the link between hillslope steepness and sediment size distribution supplied to the channel. We explore this relationship along Boulder and Fourmile Creeks, two rivers draining the Colorado Front Range. We extracted hillslope angles from digital elevation models at 8 locations of constant (granitic) lithology. Measured slopes ranged from 2 ± 0.2 to 35 ± 2.9 degrees, increasing with proximity to migratory knickzones on both channels. At each slope measurement location, we recorded the size of the 50 largest blocks with a long axis >0.5 m in a 50 m² area (10 m along-channel x 5 m up hillslope). Comparison of the sum of long axes (an effective proxy for total mass supplied) from each location on Boulder Creek against the corresponding hillslope angles reveals an abrupt transition at ~34°. Hillslopes below 34° exhibit fewer blocks over 50 cm as well as lower values of total long axis length (3 blocks and 135 cm total long axis) while hillslopes over 34° show sediment size distributions heavily skewed towards large blocks (15 blocks and 1600 cm total long axis). Fourmile Creek showed similar trends, but a low number of sample sites precludes comparison with Boulder Creek. Increases in the sum of long axes may indicate a general abundance of larger sediment or presence of a few very large blocks, both of which may significantly influence river incision. We conclude that hillslope angle influences sediment size distribution in the study channels. Complicating factors include variable fracture density and the presence of pre-existing alluvial fill in some study reaches. Our results provide first-order estimates of hillslope control on sediment supply size distribution that may be employed in modeling complex river incision processes.