GRAIN-SIZE VARIATIONS AND COUPLED CHANNEL-HILLSLOPE EVOLUTION ALONG A TECTONIC GRADIENT, BOLINAS RIDGE, CALIFORNIA

In tectonically active mountain belts, the interplay between tectonic and climatic forcing is encoded in the topography of hillslopes and channel systems through coordinated adjustment of topographic gradient(s) and erosion rates. The degree to which these adjustments reflect changes in sediment flux and caliber, however, are not well understood. In particular, long term adjustment of channel steepness (gradient normalized for contributing basin drainage area) to erosion rate is thought to reflect the influence of a threshold for erosion in the presence of a variable runoff distribution. Here, we evaluate how coordinated adjustment of hillslope gradient, erosion rate, and sediment grain size modulates the relationship between channel steepness and erosion rate gradient in rock uplift. We present a case study of a series of first-order watersheds draining to the western flank of Bolinas Ridge, California that exhibit spatial differences in both channel steepness and the ridgecrest curvature atop interfluves. Grain size distributions of channel bed sediment are characterized by coarser median grain size fractions wider variance in grain size distributions in steeper watersheds. These observations imply that transport thresholds likely co-vary with rock uplift and erosion rate. To evaluate the degree to which these variable thresholds influence channel steepness, we employ a stochastic threshold incision model combined with runoff variability obtained from discharge distributions in nearby watersheds. Our preliminary results are consistent with theoretical expectations that greater transport thresholds lead to steeper channels, at a given erosion rate, than expected from a uniform threshold. We are currently determining cosmogenic ¹⁰Be catchment-averaged erosion rates along strike of the ridge to test and refine models for the adjustment of landscape morphology to rock uplift and/or erosion rate.