UC Davis

In alluvial rivers, whose bed and banks are made up of mobile sediment, river geometry generally adjusts to just mobilize the most resistant material lining the channel. In essence, such rivers are freely formed by sediment transport processes (e.g. erosion, deposition, transport, storage). In laterally confined coarse-bedded rivers where adjustment of channel planform and gradient are more restricted this is not always the case, resulting in channel morphologies characterized by resistant boundaries and large substrates, and predominance of other modes of adjustment such as changes in bed roughness. While study of mountain river geomorphology and hydraulics has increased in last 30 years, there remains room for methodological advancement and basic science exploration, especially given availability of technological innovations in data collection, modeling, and data analysis. Thus, the goals of this dissertation were: (i) to advance methods for geomorphic and hydraulic assessment suited to mountain river settings; and (ii) improve foundational understanding of mountain river channel morphology, geomorphic processes, and process-morphology linkages. Using a 13.2-km segment of the mountainous Yuba River (Northern California) as a test bed river study site, three chapters present this work:

In Chapter 1, a procedure was developed to map sub-meter resolution large bed elements (LBEs) from a 3D point-cloud and test the hypothesis that element configurations were organized to maximize flow resistance. The procedure, which involved applying a ground classification algorithm to produce a roughness surface model and extracting LBEs with a marker controlled watershed algorithm, resulted in mapping 42,176 LBEs in the study site. Scale and discharge-dependent LBE concentration and spacing metrics quantified for multiple laterally and/or hierarchically nested spatial domains and classified using three flow-resistance based hydrodynamic regimes confirmed nearly all segment- and reach-scale LBE concentrations corresponded to a state of maximum resistance. However, disparities between concentration and spacing metrics left open questions about resistance maximization as an extremal model of geomorphic adjustment. Finer scale analyses demonstrated spatial variability of LBE configurations, but identified maximum resistance act as an attractor state toward which conditions converge. Lastly, lateral variability of LBE metrics had implications for discharge-dependent resistance.

Chapter 2 couples sub-meter resolution 2D hydrodynamic modeling with LBE mapping from Chapter 1 to present novel distributions of LBE relative submergences at multiple spatial scales and explores the dynamism of this hydraulic property across discharges and spatial domains (e.g. segment and reaches). Analysis confirmed the rate at which statistical and parametric properties changed between discharge-dependent LBE relative submergence datasets were statistically equivalent between study reaches, which we term ‘process-based similarity’. One interpretation of this consistent scaling is that it represents a dynamic equilibrium in channel adjustment toward a critical state that minimizes the variance of how resistance changes with discharge between reaches. The presented ability to account for more complete representation of bed-surface heterogeneity and the joint-distribution of local flow depths has far reaching implications. For instance, accounting for relative submergence distributions in resistance equations could improve prediction and address the limitation that a 1:1 relation exists between mean depth and mean velocity present in most resistance equations. Observation of discharge-dependent dynamism of LBE height distributions also calls into question the practice of holding roughness coefficients constant, and the ability to map individual LBEs provides a sensible method for parameterizing spatially variable roughness lengths scales.

Finally, Chapter 3 presents an investigation into local topographic controls and morphodynamic processes involved in formation and/or persistence of morphological unit scale fluvial landforms (MU) and addresses five scientific questions about mountain river MUs and their hydro-morphological (HM) variables. The study applies a rigorous top-down classification followed by bottom-up analysis experimental design whereby a standard method involving meter-scale 2D hydrodynamic modeling and a baseflow hydraulics decision tree were used to classify and map nine spatially explicit baseflow in-channel MU types in the study site. Discretizing the study site into cross-sectional polygons a total of 2539 cross-sections were identified as being dominated by a single MU type and a diverse set of 18 HM variables, representing an array of possible hydraulic and geomorphic controls on MU formation and/or maintenance, were measured at these cross-sections. Cumulatively, study results develop holistic descriptions of HM variable conditions where certain MUs and/or groups of MUs occurred, interpret processes involved in the formation/persistence of these MU types, and provide inference on how HM variables exert deterministic control.