Controls on Upland River Channel Geometry

Upland (high-gradient, shallow-bedrock) rivers are prevalent in mountainous landscapes and are widely appreciated for their aesthetics and hydropower potential, amongst other uses. Despite the abundance and appreciation of upland rivers and their form, the controls on upland river channel form are poorly understood. For instance, channel flow parameters are often used to define quantitative hydraulic geometry relationships that describe the form of gravel-bed, alluvial rivers; the limitations of hydraulic geometry relationships to describe upland systems are not well-studied. Variations in upland river form are likely controlled in many locations, for example, by shallow bedrock. Additionally, river classification systems are often used to categorically describe morphological differences amongst alluvial rivers, but are seldom applied to or developed for upland river channels. In this study, data from upland and gravel-bed alluvial rivers worldwide are used to examine the overlap in hydraulic-geometry theory and river classification systems, and thus also to extend existing classification systems to include dimensionless values of bankfull channel width and mean flow depth. Froude number dependence on slope, as unveiled through characterization of rivers by substrate, e.g. bedrock, mixed bedrock-alluvial, or alluvial, illustrates the necessity of different parameterizations of friction for alluvial and upland channels. Friction in mixed and bedrock upland channels is effectively parameterized using critical Froude number scaling, which reflects minimized downstream energy propagation; stable, gravel-bed alluvial rivers are well constrained by a critical Shields number closure where friction is proportional to the relative submergence of characteristic bed sediments. The proposed hydraulic geometry-channel classification system offers insights toward river restoration design, landscape evolution modeling, and paleohydraulic reconstruction.