Width variability controls on and channel slope response to spatial patterns of macro-roughness elements in mountain rivers

Abstract:

Macro-roughness elements such as boulders and bedrock outcrops, collectively referred to as large bed elements (LBEs), are a distinctive component of mountain rivers whose spatial patterns strongly influence hydrodynamic and morphodynamic channel properties. Linkages between hillslope processes and LBEs are reasonably documented but less is known on how morphological properties of the river corridor influence or respond to spatial patterns of LBEs. For instance, high LBE concentrations have been linked to steeper than expected river profiles supporting a process of LBE mediated fluvial incision that requires further field testing. This study addresses these gaps by testing relationships between morphological metrics and spatial patterns of in-channel LBEs in a mountain river at several spatial scales. All analysis was completed in a 13.2-km segment of the mountainous Yuba River (Northern California). A complete census of over 42,000 LBEs were mapped from terrestrial and bathymetric airborne LiDAR data using a tree canopy segmentation algorithm and longitudinal series of cross-sectional stage-dependent in-channel LBE concentrations were generated in 3 m increments along the river corridor. These data were tested against four series of river valley/channel morphological metrics: channel gradient, channel width, width-to-depth ratio, and valley width-to-baseflow-width ratio extracted at the same scale from a meter-scale digital terrain model and 2D hydrodynamic modeling. Preliminary findings show cross-section scale relationships between in-channel LBEs and all test metrics were highly variable and had weak linear correlations. At the reach scale there was a positive linear relationship between LBE concentration and channel slope, which corresponds with previous findings. Valley width-to-baseflow-width ratio was the only other variable with a strong connection. This variable’s positive relationship with concentration supports congregation of LBEs in confined sections of the river valley with high hillslope connectivity. Patterns of LBEs and the scale-dependent relationships with morphological variable in the study site help reveal potential landscape controls and response to LBEs, however further study is needed to elucidate how interacting processes mediate LBE dynamics.