Topographic steering in mountain rivers with thousands of large bed elements

Morphological control of river hydraulics, or ‘topographic steering’, influences the spatial patterning and persistence of aquatic habitats and channel-change processes. While imperative to addressing questions or problems in river engineering, fluvial geomorphology, and aquatic ecology the study of topographic steering in natural rivers is still limited, particularly amidst the landform complexity of mountain rivers. This study presents new theory, methods, and findings for investigating landform structure, hydraulics, and mechanisms by which topographic steering asserts control in mountain rivers. Using a novel combination of point-cloud processing and topographic differencing 52,926 individual boulders and bedrock outcrops were mapped within a 13.2-km segment of the mountainous Yuba River (Northern California). Employing a 15% cover threshold these large bed elements (LBEs) were identified as an important source of landform non-uniformity. Coupled with outputs from two-dimensional (2D) hydrodynamic modeling and an original flow pattern analysis framework, LBEs were also revealed as significantly contributing to highly non-uniform flow patterns classified during discharge simulations ranging from baseflow to a ~3.5-year flood. New metrics on flow pattern hydraulics and spatial persistence outline a spectrum of hydraulic-morphodynamic mechanisms describing stage-dependent spatial stratification of erosional and depositional tendencies. Within this continuum three hydraulic-morphodynamic conceptual models (“chutes-and-traps”, “stage-dependent dynamic mobility”, and “variable-scale topographic steering”) are proposed and tested for their relative roles in the Yuba River study site. Findings show the site is characterized by a mechanism where changing flow patterns provide a template for many combinations for sediment mobilization and deposition important to the sediment balance of the supply-limited study site.