EVALUATING CHANNEL-HILLSLOPE COUPLING ALONG AN EROSION RATE GRADIENT, BOLINAS RIDGE, CALIFORNIA

The associations among rock strength, sediment caliber, channel morphology, and the stochastic variability of discharge are expected to strongly modulate the adjustment of channel profiles to uplift and erosion rate. Although sediment grain size is canonically considered to reflect the rate of erosion in upland watersheds, quantifying variations in grain size with differences in erosion rate has proven challenging. Here, we examine the nature of hillslope-channel coupling along a gradient in topography and (inferred) erosion rate along Bolinas Ridge, California. Numerous first-order watersheds drain the western flank of the ridge, above the San Andreas fault, and exhibit systematic variations in topographic relief from north to south. Analysis of high-resolution topographic data acquired from lidar surveys reveal that the curvature of interfluves increases nearly linearly along the ridge, consistent with an expected variation in erosion rate. Smooth and concave-up channel profiles also steepen toward the south; variations in channel steepness (a measure of gradient normalized for upstream watershed area) appear to increase non-linearly from ~15 m^0.9 at the north end of the ridge to ~80 m^0.9 at the south. Although a few channels exhibit knickpoints separating low-gradient upstream reaches from steeper downstream reaches, these appear limited to the central portion of the transect. We measured the grain-size distribution of sediment in the active channel using photogrammetric techniques; results reveal systematic increases in both median grain size and variance of the distribution with increasing topographic relief. We are currently evaluating the utility of a stochastic-threshold stream power model to account for the role of variable transport thresholds that arise from the coupling between erosion rate and sediment grain size.