Year of Award

2024

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Andrew C. Wilcox

Commitee Members

W. Payton Gardner, Douglas Brinkerhoff, Jean Dixon, Brendan Murphy

Keywords

bedload, mountain rivers, sediment connectivity, sediment transport

Abstract

Sediment connectivity is a framework for conceptualizing how sediment moves from source to sink across landscapes. Quantifying connectivity is challenging due to variation in sediment detachment and transport processes and their rates across different process domains. Within the Bitterroot River basin of Western Montana, process domains differ based on glaciation history, and resulting morphological differences; the Bitterroot Mountains were glaciated during the Pleistocene, and the Sapphires/Southern Bitterroots were not. Quantifying longitudinal connectivity requires estimating sediment transport from reach to reach throughout drainage basins. Previously, transport equations suitable for application in mountain rivers have been limited. I present a new transport equation for estimating fractional bedload transport in poorly-sorted, coarse-bedded rivers. This framework builds on existing fractional transport frameworks, and uses new methods to estimate the fraction of boundary shear stress acting on mobile grains, and convert pebble counts to areas to better represent relative exposure to the flow. The equations produce results in Bitterroot Basin streams that compare well with measurements of sediment yield for the region. In order to apply the fractional sediment transport formula across entire drainage networks, I developed a new algorithm for extrapolating grain size measurements across basins. Applying the algorithm to various sub-basins of the Bitterroot River produced predictions within ~5% of measured grain sizes on the phi grain size scale. For quantifying hillslope-channel connectivity, I developed a deposit evolution model that simulates fluvial recruitment and transport of sediment from mass wasting deposits, and the resulting morphological change through time. Debris flow deposits affect sediment yield and connectivity for years to decades in the study area, depending on the size of the deposit and slope of the channel. Rockfall deposits serve as lasting sediment traps, reducing longitudinal connectivity. Finally, I developed a framework for quantifying hillslope-channel connectivity and longitudinal connectivity for each reach of a drainage network. Application in the Bitterroot showed that hillslope sediment delivery processes are important drivers of variation in longitudinal connectivity. In previously glaciated basins, rockfall delivers large clasts to channels that remain immobile, whereas sediment delivered via debris flows in basins that were not previously glaciated is largely transportable by channels.

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© Copyright 2024 Jordan Thomas Gilbert