A Field Test Of The Influence Of Grain Size In Determining Bedrock River Channel Slope
Interpreting spatial patterns in rates of fluvial incision from river channel elevation long profile data requires an assumption that tectonic uplift rate governs river channel slope. However, application of the most mechanistically explicit description of river incision [Sklar and Dietrich,2004] suggests that sediment flux and sediment grain size, not rock uplift rate, control river channel slopes in many settings. Because it is usually difficult to independently constrain sediment supply, tectonic interpretations of river elevation long profiles are necessarily uncertain. Here we exploit a natural experiment in Boulder Creek, a ~ 30 km^2 drainage in the Santa Cruz Mountains, CA USA in order isolate the affect of grain size and relative sediment supply on river channel slope in an actively uplifting landscape along a restraining bend in the San Andreas Fault.
A single prominent knickpoint exists near the midpoint of Boulder Creek, separating a 6.8 km long region of low slope(~0.8%) from a steeper(~2.6%) 4.8 km reach along the lower portion of the channel . Mapping and field observations reveal that this knickpoint does not coincide with any lithologic or tectonic boundaries; the channel cuts weak sedimentary rock for it length. In addition, longer wavelength changes in rates of rock uplift due to the bend in the San Andreas fault near Boulder Creek are negligible over the relative small size of Boulder Creek's catchment. Instead the knickpoint coincides with the location of the first tributary that taps a source of resistant, granitic sediment that is not found in the upstream reaches of Boulder Creek. Field observations indicate that coarse granitic bedload is sourced by debris flows and introduced by a series of tributaries draining into the steep lower reaches of Boulder Creek. The knickpoint marks a transition in median grain size from ~2cm upstream of the knickpoint compared to an average of ~18cm downstream of the knickpoint. Additionally, upstream of the knickpoint , Boulder Creek is characterized by potholes and sculpted bedrock, consistent with sediment-starved conditions.
The observation that bedrock channel slope changes are not well correlated with patterns in rock uplift supports Sklar and Dietrich's (2006) theoretical result that modest rates of rock uplift do not significantly influence river profile slopes. Based on this result and the clear correlation of channel slope and sediment supply along Boulder Creek, we chose to ignore rock uplift rate and instead explore the relative roles of grain size and sediment flux in influencing profile slopes along Boulder Creek. Using field surveys of grain size and high flow depth, we calculate that ~10% of the slope above the knickpoint and ~30% of the slope below the knickpoint is related to maintenance of the channel at the threshold for sediment motion. This implies that ~90% of the slope above the knickpoint and ~70% of the slope below the knickpoint is due to the excess stress that is required to move the coarse sediment load. This would imply that other factors including sediment supply and bed roughness can be significantly more important than thresholds of coarse sediment motion for setting channel slope in mixed bedrock-alluvial systems.