ELUCIDATING CHANNEL-HILLSLOPE COUPLING ALONG A TECTONIC GRADIENT: CO-VARIATION IN EROSION RATE AND GRAIN SIZE SETS CHANNEL FORM

A primary challenge to the inference of uplift rates and patterns from topography emerges from the interplay among sediment caliber, precipitation/runoff distributions, and thresholds for channel incision. The evident dependence of grain size distributions on rock fracture density suggests the potential for incision thresholds to co-vary with rock uplift/erosion rate. Here, we present new analysis of the relationships among channel steepness, landscape topography, grain size, and erosion rate along an apparent gradient in differential rock uplift in coastal California.

Previous analysis of landscape topography along Bolinas Ridge suggests that both channels and hillslopes in low-order watersheds draining the flank of the ridge are adjusting to spatial gradients in rock uplift rate. Field observations of grain size distributions reveal a systematic correlation between sediment caliber, interfluve curvature, and channel steepness. Steeper watersheds transport sediment with coarser median grain sizes, implying that thresholds for sediment transport and erosion in channels co-vary with topography. Analysis of hydroclimatic regimes from gauged watersheds throughout the region reveals stretched exponential discharge distributions, and these runoff characteristics are used to drive a stochastic river incision model. The results of this analysis predict a non-linear scaling between channel steepness and erosion rate, but differences in the transport threshold drive the effective scaling to be linear. Comparison of these results to 1) erosion rates along the transect predicted from interfluve curvature and 2) direct measurement of 10Be concentrations in modern channel sediment suggests that erosion rates along Bolinas Ridge vary from ~50-100 m/Myr to >~300 m/Myr. Overall, our results demonstrate how variations in sediment grain size shed from hillslopes may drive the adjustment of topography to variable uplift and suggest the need to understand the initial generation of grain sizes on rapidly eroding hillslopes.