Analyzing bed and width oscillations in a self-maintained gravel-cobble bedded river using geomorphic covariance structures

Abstract

This paper demonstrates a relatively new method of analysis for stage dependent patterns in meter-scale resolution river DEMs, termed geomorphic covariance structures (GCSs). A GCS is a univariate and/or bivariate spatial relationship amongst or between variables along a pathway in a river corridor. Variables assessed can be flow independent measures of topography (e.g., bed elevation, centerline curvature, and cross section asymmetry) and sediment size as well as flow dependent hydraulics (e.g., top width, depth, velocity, and shear stress; Brown, 2014), topographic change, and biotic variables (e.g., biomass and habitat utilization). The GCS analysis is used to understand if and how the covariance of bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel-cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges through a suite of spatial series analyses on 6.4 km of the lower Yuba River in California, USA. A key conclusion is that the test river exhibited positively covarying and quasi-periodic oscillations of bed elevation and channel width that had a unique response to discharge as supported by several tests. As discharge increased, the amount of positively covarying values of bed elevation and flow-dependent channel top width increased up until the 1.5 and 2.5 year annual recurrence flow and then decreased at the 5 year flow before stabilizing for higher flows. These covarying oscillations are quasi-periodic scaling with the length scales of pools, bars, and valley oscillations. Thus, it is thought that partially confined gravel-cobble bedded alluvial rivers organize their adjustable topography with a preference for covarying and quasi-periodic bed and width undulations at channel forming flows due to both local bar-pool mechanisms and non alluvial topographic controls.