Abstract

Rivers are remarkably consistent as their geometry, width, depth, and slope change systematically downstream in response to differences in fluid discharge and sediment transport throughout a channel. Rivers move both water and sediment through terrains that depend on these downstream geometric adjustments influencing water quality, ecology, infrastructure, channel morphology, and landscape evolution at increasing timescales respectively. Rivers thus influence all aspects of life and define our spatial landscapes that shape boundaries, whether physical, politically, or socially. Nevertheless, a river is dynamic and does not adhere to human-defined boundaries, making it vital that we understand how these features change over time and space. The temporal and spatial adjustments result in measurements that are often noisy, complicating our understanding of rivers. Additionally, we are also facing an unprecedented time of accelerated warming which directly influences hydroclimates, further muddying our understanding of how rivers may adjust in the future. Behind every gravel river, sediment lines the bed and the banks of the channel, indicating that for any change to occur in this river, sediment must be entrained and deposited. A framework is presented here in which the mobilization of grains dictates channel geometric change and by understanding how different thresholds of mobilization can be reflected in channel geometry, we can further recognize how perturbations to hydroclimates will influence river morphology. These thresholds, which are referred to as erosion thresholds, can change under a variety of conditions and require that fluid shear stresses exceed these thresholds for sediment transport to occur. By realizing the role in which erosion threshold variability regulates river channel size and shape under the basis of near-threshold channel theory, river noise, be it measurement noise, downstream variability, or the noise of the river itself, can be used to uncover river processes by looking at different spatial scales. By exploring the changes in channel geometry at the scale of the longitudinal profile, downstream elevation with distance, equilibrium conditions can be determined. Yet, there are variations in channel form over the longitudinal profile of a river, specifically at states of disequilibrium, and the variations represent different processes that allow for localized unique geometry to exist amongst these broader downstream trends. Downstream channel behavior is best described by hydraulic geometry scaling or how channel geometry evolves with fluid discharge but fails to capture the nuanced variability of channel geometry. By comparing deviations or noise from these scaling relationships, they reveal that channels in equilibrium, concave in form, and disequilibrium, knickpoints present, exhibit unique distinct coupling between bankfull width and channel slope. In segments that are steeper than the central tendency of hydraulic geometry would predict, knickpoint channels are narrower than expected while concave channels are wider. Additionally, these geometric measurements express a new downstream behavior not captured in large data compilations in which bankfull width becomes insensitive to channel slope. These unique behaviors in width and slope suggest that transport capacities can be maintained for concave channels while erosive power, likely due to increased erosion thresholds, must increase knickpoint channels. The exploration at a river’s longitudinal scale can inform us how changes in the channel profile, such as though climate perturbations, can directly influence how sediment is transported downstream and is discussed in Chapter 2 titled High-Resolution Channel Geometry Reveals Contrasting Styles of Gravel River Adjustment. Exploring the spatial scale at which individual sediment grains move within a river defines our understanding and estimates of sediment transport. These motions are controlled by erosion thresholds of distinct grains and are the basis for setting the shape, width to depth, of a river. These thresholds are not uniform and are influenced by many controlling factors such as river slope or low flow durations. Increasing channel slope can decrease relative submergence or the exposure of grains, changing how drag and lift forces interact with these grains while increasing low flow durations pivot grains into more stabilized arrangements without transporting them. By exploring the combined attributions of these conditions, noisy sediment flux decreases over a range of slopes spanning from 0.005 to 0.07 m/m for longer low flow durations. The extent to which flux decreases depends directly on channel slope as steeper slopes can stabilize riverbeds more rapidly for a given flow duration. Grain scale controls on the mobilization of sediment under changing slope and low flow durations is discussed in Chapter 3 Slope-dependent riverbed strengthening and the evolution of the sediment mobility in gravel-bed rivers and provides guidance on how different segments of a river channel may adjust under a changing hydroclimate. The cross-sectional scale of a river determines local channel stability and the conveyance of sediment. The amount of sediment that moves through a channel cross-section determines if a river needs to widen, deepen, or re-grade to maintain equal sediment flowing over decadal time periods, yet measuring sediment transport in real time is notoriously challenging. Seismological noise from a river offers the ability to potentially capture sediment transport at a continuous rate, often through hysteresis behavior that the noise exhibits with fluid discharge. Clockwise hysteresis tends to indicate sediment transport on the rise of an event or season while counterclockwise suggests it may occur on the falling limb. Turbulent eddies also produce these hysteresis behaviors and indicate potential differences in hydraulics. By quantifying hysteresis, for both bedload and turbulent energy, at multiple seismic nodes along a mixed bedrock alluvial river during a multimodal flood event, there are noticeable differences in the timing of sediment transport and flow hydraulics that depend on the magnitude of a flood event. Additionally, hysteresis between these energy types becomes uncoupled at larger magnitude flood events yet return to similar directionality under more moderate flood events. Both bed material and pre-event bankfull width influence hysteresis for both energy types by influencing sediment supply and setting channel adjustments. The unique conveyance of these fluvial signals are discussed in Chapter 4 Morphologic controls on seismic hysteresis along a mixed-bed river during a large flood sequence and demonstrates how flood magnitude, which can vary under different hydroclimate conditions, coupled with morphologic controls can alter sediment transport responses downstream. By exploring the noise of rivers at different spatial scales under the fundamental connection that variable erosion thresholds influence channel geometry, this framework can be used to explore how rivers adjust under changing hydroclimates. The longitudinal scale exposes downstream geometric variability that results in different modes of adjustment for gravel rivers in equilibrium or disequilibrium. Sustained climate perturbations along a section of a channel profile will shift channels out of equilibrium forcing geomorphic adjustments that may be represented by different geometric coupling. Noisy sediment flux at the grain scale uncovers a dependency on channel slope during low flows for granular strengthening which happens most rapidly at steeper slopes. The length in which low conditioning occurs, which depends on local hydrology, directly influences the mobility of sediment at steeper and gentler slopes. The tangible seismic noise produced from a river reveals that the timing of sediment transport and fluid hydraulics depends on flood magnitudes, channel morphology, and previous flood events. Changes in water balance inputs can directly correspond to size and timing of floods which can modulate when and where sediment is transported in a river reach. Thus, while rivers are noisy, within the noise are processes that can be disentangled to inform future communities and ecosystems.

Committee Chair

Claire Masteller

Committee Members

Bronwen Konecky; Dougal Hansen; Douglas Wiens; Matthew Rossi

Degree

Doctor of Philosophy (PhD)

Author's Department

Earth & Planetary Sciences

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

8-6-2026

Language

English (en)

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Geomorphology Commons

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