Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Gregory E. Tucker

Second Advisor

Robert S. Anderson

Third Advisor

Suzanne P. Anderson

Fourth Advisor

Cameron W. Wobus

Fifth Advisor

Harihar Rajaram

Abstract

Climate shapes the surface of the earth through processes both torrential and mundane. I quantify the geomorphic impact of climate fluctuations in the Colorado Front Range by combining field work and numerical modeling on a variety of spatial scales. This research ranges from determining how water delivery controls the spatial development of saprolite on sub-alpine hillslopes, to how sediment delivery to streams causes channels to migrate across a landscape.

Through the delivery of water in snowmelt, climate should govern the rate and extent of saprolite formation in snow-dominated mountain watersheds, yet the mechanisms by which water flows deeply into weathered rock are largely unexplored. Measurements of snow pack thickness and soil moisture reveal strong contrasts between north- and south-facing slopes in both the timing of melt-water delivery and the duration of significant soil wetting in the shallow vadose zone. Results from a 2D numerical model of vadose zone dynamics suggest that thicker soil and more deeply weathered rock on north-facing slopes reflect greater moisture supply and weathering intensity.

Sediment that is produced on the hillslopes eventually arrives in stream channels. In the summer of 2013, I measured grain size, lithology, and channel geometry on several streams. Shortly after the conclusion of this field campaign, record-shattering rainfall caused severe flooding in the Front Range, including all of the study streams. Following the flood, half of the originally sampled sites were re-surveyed. This data set offers a unique opportunity to study empirically how a torrential flood event changes the size and composition of channel bed material and how the shape of the channel itself changes.

Modulation of sediment supply or transport capacity associated with climate change related to glacial-interglacial cycles has been suggested as a possible driver for the repeated aggradation and abandonment of strath terraces that flank the Front Range. In this study, I use a landscape evolution model to determine whether changes in glacially driven sediment flux, changes in hillslope sediment flux or changes in transport capacity of the stream, in isolation or in combination, are sufficient to explain the observed rates and patterns of terrace formation and abandonment. The models indicate that i) in the absence of a large addition of sediment to the streams, variations in stream power are necessary to allow channel aggradation and the planation of bedrock surfaces, and ii) increased sediment flux from hillslopes is necessary to match observations of increased denudation rates during deposition of terrace-capping gravels.

Strath terraces are evidence of periods of time when the lateral erosion of bedrock channels dominated over vertical incision in bedrock channels. I present a physically-based theory for the lateral migration of bedrock channels and explore climate controls on lateral erosion rates and extent of valley widening. The model predicts that weaker bedrock results in wider bedrock valleys and more channel mobility, which is a fundamental factor for developing and maintaining a bedrock valley that is several times wider than the channel it holds. Increased channel mobility and wider flat bottomed valleys in the model under transport-limited conditions suggest that sediment cover on the bed is an effective way to slow vertical incision and amplify the effect of lateral erosion. This theory for the lateral erosion of bedrock channel walls and the numerical implementation of the theory in a catchment-scale landscape evolution model is a significant first step towards understanding the factors that control the rates and spatial extent of wide bedrock valleys.

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