Date of Award:

2017

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Joel L. Pederson

Abstract

The Snake River flows across the dynamically uplifting hotspot plume of the Yellowstone region, cuts through the Snake River Range and ultimately enters the lowlying eastern Snake River Plain. Thermal and mantle-dynamic uplift around Yellowstone has been recorded by short-term geodesy and modeled by geophysicists, but measurements over Quaternary timescales and an understanding of how that uplift influences regional incision are absent. The Snake River is the only regional river that crosses the uplifting Yellowstone Plateau and flows into the subsiding eastern Snake River Plain (SRP), and provides an opportunity to investigate both ends of the phenomenon on the tailing margin of the Yellowstone region.

This thesis consists of two related studies conducted in Alpine Canyon of the Snake River. The first is a study of fluvial terraces and steepness patterns along the Snake River considering the spatial distribution of bedrock or varying hardness and resistance to erosion and in the context of regional tectonics. This study uses surficial mapping, optically stimulated luminescence (OSL) dating, bedrock strength measurements, and steepness analyses of the mainstem Snake River and tributary drainages. Results include the first incision rate estimates for the southwestern part of the Yellowstone hotspot region and a discussion of the possible sources of baselevel fall along the Snake River.

The second study documents the transitions between bedrock and alluvial channels in the study area and evaluates morphometric and transport capacity thresholds between these reaches. Alluvial bed-cover mapping with a side-scan sonar along with channel morphometric data, clast-counts, and sediment transport estimates allow us to explore what controls these two fundamental channel types.

Results confirm that the Snake River has relatively fast incision rates for the interior western U.S. and that the Snake River is adjusting to an actively deforming landscape. Additionally, our dataset provides field documentation of the magnitude of bedrock-alluvial transitions and may be valuable for parameterizing landscape evolution models or assisting in the restoration of reaches that are in disequilibrium due to changes in land use or climate. This study will hopefully inspire future studies of tectonism and landscape evolution of the Yellowstone hotspot region.

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