Date of Award:

8-2013

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Watershed Sciences

Committee Chair(s)

John C. Schmidt

Committee

John C. Schmidt

Committee

Joseph M. Wheaton

Committee

Patrick Belmont

Committee

Karin M. Kettenring

Committee

David Rosenberg

Abstract

The accurate prediction of river channel width remains a fundamental area of investigation in the field of geomorphology. River managers and scientists are interested in understanding how a channel will respond to environmental perturbations such as altered runoff patterns from climate change, a new dam, or a pulse of sediment from a landslide. Increasingly, studies that focus on this question acknowledge the importance of accounting for the vegetation that lines the river banks. For this dissertation, I strove to identify some of the primary ways by which vegetation affects channel width.

At a fundamental level, vegetation influences the size of a channel by altering the depth and velocity of flowing water and the transport of sediment. To account for this, I developed a novel method that links small scale interactions among water, sediment, and plants to the larger scale over which channel width is evaluated. While this multi-scalar (i.e., linking multiple scales) methodology may be generalized to any type of vegetation community, I applied it specifically to stands of the non-native riparian shrub, tamarisk that invaded the rivers of the southwestern US during the past century.

I focused my analyses on the lower Yampa River in western Colorado. Tamarisk expansion along the free-flowing Yampa occurred in the absence of modifications to the delivery of water or sediment from upstream that often occur as a result of dams or water development. Where tamarisk established, the channel narrowed. Thus, the Yampa is a unique environmental setting. Without changes to the water and sediment, adjustments to channel form may be linked to an altered vegetation community. A careful reconstruction of the timing, style, and pattern of channel and vegetation changes that have occurred on the Yampa during the past 50 years informed us on the important processes by which the expansion of vegetation affects channel width. Specific attention was paid to the relative importance of commonly occurring vs infrequent, large floods in driving these processes.

Tamarisk establishment enhanced not only sediment deposition that leads to channel narrowing, but also to new vegetation establishment. Plants increased the friction in the channel, thus decreasing water velocity close to plants. Low velocity areas became susceptible to further vegetation encroachment, particularly if they did not have high velocities for a series of ~4 or more years. As vegetation encroached and changed the shape of the channel, the importance of common and large floods, for vegetation establishment and sediment transport, changed. Application of this process-based understanding to future flow regimes will help managers anticipate locations along the channel that are susceptible to vegetation encroachment and changes to channel width.

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Geomorphology Commons

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