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Date of Award

Fall 2024

Document Type

Masters Thesis

Department or Program Affiliation

Geology Department

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Pfeiffer, Allison

Second Advisor

Clark, Douglas H., 1961-

Third Advisor

Walowski, Kristina

Abstract

In the Pacific Northwest, more than half of catastrophic mass wasting events occur on volcanic slopes. As such, these volcanic landscapes serve as major contributors of sediment to river valleys downstream. Despite decades of research on mass wasting event mechanics and hazards, we lack a strong predictive framework for the downstream effects of sediment pulses in rivers. The grain size distribution (GSD) and clast abrasion rate of mass wasting deposit sediment can place strong controls on the downstream fate of these sediment pulses. Prior work completed on the Suiattle River at Glacier Peak, a stratovolcano in Washington State, constrained the GSD, lithology, and abrasion variability of a volcanic mass wasting deposit and demonstrated that much of the coarse material abrades to fines in the first 10s of km of transport. I build on this effort by quantifying the clast strength and GSD of four other large volcanic mass wasting deposits in the region with different volcanic source material: Lillooet River at Mt. Meager, White River at Mt. Rainier, Kautz Creek at Mt. Rainier, and Salt Creek at Mt. Adams. Results show that measured coarse material GSDs vary substantially between deposits, with median grain sizes between 0.02 m and 0.09 m and the 95th percentile grain size, D95 between 0.20 and 0.83 m. Each deposit contains a variety of volcanic rock types, along with minor components of plutonic and metamorphic in some deposits. These rocks span a range of vesicularity and hydrothermal alteration, two factors I find are associated with weaker rocks. Sediment strength is highly variable within each measured deposit, with calculated abrasion rates (1/km) varying over 4 orders of magnitude. Modeling demonstrates that while calculated bed material abrasion is substantial (>51% over 100 km of transport) for all of these deposits, modeled coarse sediment loss exceeds 73% over 100 km of transport for the deposits with higher vesicularity and hydrothermal alteration. To provide context for these findings, I compare volcanic rock strength data to a global compilation and find that the deposits interrogated in this study are within the common range for volcanic rocks. Importantly, sedimentary rocks tend to be substantially weaker, implying that the effects of pulse attenuation would be even greater than what I find at my sites. Bed material abrasion is an important, and likely underestimated, control on the source-to-sink coarse sediment budget of rivers in many settings.

Type

Text

Keywords

Pacific Northwest, Catastrophic mass wasting events, Debris flow, Lahar, Sediment pulse attenuation, Abrasion rate, Volcanic landscapes, Hydrothermal alteration, Vesicularity, Grain size distribution, Density, Rock strength, Downstream transport, Mt. Meager, Lillooet River, Glacier Peak, Suiattle River, Mt. Rainier, White River, Kautz Creek, Mt. Adams, Salt Creek

Publisher

Western Washington University

OCLC Number

1477966760

Subject – LCSH

Mass-wasting--Northwest, Pacific; Debris avalanches--Northwest, Pacific; Lahars--Northwest, Pacific; Volcanology--Northwest, Pacific; Hydrothermal alteration--Northwest, Pacific

Geographic Coverage

Northwest, Pacific

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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