Bedrock channel morphology in relation to erosional processes

Abstract

Bedrock channel morphology reflects the interactions between erosive processes and the resistance of the channel substrate. The controls on these interactions change with spatial scale. Mineralogy, exposure age of the substrate, and local heterogeneities are particularly important in controlling substrate resistance at the micro scale (mm to cm). Substrate discontinuities created by bedding, joints, and lithologic contacts become progressively more important at the meso scale (cm to m), whereas regional structure and baselevel history may dominate substrate resistance at the macro scale (m to km). In a similar manner, turbulent fluctuations that create localized abrasion and cavitation are more important at the micro and meso scales, whereas longitudinal patterns of unit and total stream power exert a stronger influence on channel morphology at the macro scale. Most studies of bedrock channel morphology have described meso-scale erosional features. In the absence of direct measurements, investigators have inferred both the erosive processes that produced the observed features, and the controls on the location of the features. Fluvial erosion of bedrock may occur via; (1) corrosion, or chemical weathering and solution, (2) corrasion, or abrasion by sediment in transport along the channel, or (3) cavitation and other hydrodynamic forces associated with flow turbulence. Very few direct measurements of rate exist for any of these erosive processes. Bedrock channel morphologies may be divided into multiple or single flowpath channels, and subdivided on the basis of sinuosity, uniformity of bed gradient, and uniformity of erosion across a cross section. These categories may be used to infer dominant erosional processes and relative rates of erosion, but we cannot yet predict the occurrence of specific channel morphologies as a function of driving and resisting forces. In this context, the traditional assumption that substrate dominates bedrock channel morphology may be too restrictive.