High-Resolution Field Measurements of Water and Bed Surface Profiles for two Bedrock Downsteps on the Upper South Fork American River, CA

A newly developed river survey system designed for high-velocity flows and complex channel topographies was used to obtain high-resolution measurements of rapidly varying water and bed surface profiles for two bedrock-controlled downsteps on the upper South Fork American River, CA. Point measurements ranged over a 2.4-7.9 m² surface area with an average point density of 46 points per square metre (pts/m²). Point data were then interpolated to develop digital terrain models (DTMs) for each profile. Finite difference analysis along several streamwise and transverse transects of all profiles indicates macroscale (10⁰ - 10¹ m) and microscale (10^-1 - 10⁰ m) variability as an important unit of variation. Supercritical water surface profiles exhibited rapidly varying planar, convex, and concave shapes, while subcritical water surface profiles exhibited gradually varying planar shapes. For all profiles, hydraulic jumps were represented by a topographic depression in the water surface. Bed surface profiles exhibited highly complex local channel morphologies with rapidly varying and undulating topographies upstream of hydraulic jumps, and scour holes, furrows, undulations, and coarse alluvial elements below. Further, flow depth varied significantly over the surface area of each downstep, with coefficients of variation (CV) ranging from 48-51%. Systematic assessment of local water surface slopes and bed surface slopes emphasize water surface and bedform obliquity, as well as significant deviations from reach-averaged values. Further study will emphasize the importance of stage-dependent changes in the 3D-flow field and the role of mutual adjustment between transcritical flow structures and bedrock channel form.