Tectonic and lithologic controls on bedrock channel profiles and processes in coastal California

Abstract

Recent theoretical models suggest that topographic characteristics of bedrock channels are products of interactions among tectonics, substrate resistance, and the climatically modulated erosive ability of the river. The degree to which these factors influence the form of channel profiles is poorly quantified at present. Here we investigate bedrock channels developed across the southern flank of the Santa Ynez Mountains, California. Uniform climate and systematic variations in lithology and rock uplift rate along the range allow comparison of channel morphology between (1) channels eroding rocks of uniform and nonuniform strength and (2) channels experiencing differences in tectonic forcing. We combine field observations, surveys, and analysis of digital data to determine topographic and hydraulic characteristics of bedrock channels. At a constant rock uplift rate, streams flowing from resistant to less resistant bedrock exhibit highly concave profiles and increased gradients along lower reaches relative to channels developed in uniform bedrock. These effects are interpreted as responses to (1) an increase in substrate resistance to channel incision in the upper reaches and (2) transport‐limited gradients along lower reaches. Comparisons of channels developed across uniform lithology but experiencing an approximately sevenfold difference in rock uplift rate reveal an approximately twofold increase in gradient and an approximately threefold decrease in width. In this landscape the combined channel adjustments of gradient and width are consistent with a fluvial incision model in which channel incision rate is linearly proportional to mean bed shear stress.