Date of Award

12-2012

Degree Type

Report

Degree Name

Master of Science (MS)

Department

Watershed Sciences

First Advisor

Joseph Wheaton

Abstract

Beaver, their dams and associated networks of dens, side-channels and pools have a profound influence on habitat heterogeneity and the complexity of the environments they occupy. The purpose of this paper is to illustrate the hydrologic and geomorphic interactions between beaver dam establishment and the greater ecosystem as well as quantify the potential geomorphic changes following beaver dam failure and the influence those changes have on the riparian and fluvial ecosystem in a semi-arid environment.

I use a case study of beaver dam breaches in a small unregulated stream, Bridge Creek, in eastern Oregon to illustrate the concepts. Dam breaches are evaluated in two separate reaches of Bridge Creek: the Upper Owens Reach where a dam failed, which was reinforced with post lines as part of an experimental restoration project; and the Boundary Reach where two natural beaver dam failures were recorded.

Given Bridge Creek’s position in the Columbia River system below most of the major mainstem dams, it is an important Middle Columbia Steelhead (Oncorhynchus mykiss) fishery. The creek is currently degraded and incised through quaternary alluvium with highly simplified in-stream habitat, which is thought to be limiting steelhead production. The riparian corridor is very limited and homogenized due to channel incision and resultant loss of floodplain connectivity.

To aid in the quantification of erosion and deposition and the subsequent influence on fluvial geomorpholoy following beaver dam failure and abandonment high resolution repeat topographic data were collected using a combination of Total Station, ground-based LiDaR, and rtkGPS surveys. The Geomorphic Change Detection software was then used to conduct DEM of difference calculations distinguishing changes due to noise from those due to geomorphic processes. Finally, I applied a mask for geomorphic interpretation of the DoD to segregate the sediment budget spatially to interpret what the changes mean structurally.

At Upper Owens, a pilot treatment site of the restoration, the DoD shows net deposition the first year followed by two years of net erosion. Still, the channel complexity of the reach increased considerably, following reoccupation of the dam site, reinforcement of the dam, expansion of the dam, and the partial breach. The homogenous plane bed morphology transformed into a complex mix of pools, point bars, mid channel bars and vegetated islands. Whereas along Boundary, a control reach, the results of the DoD show net deposition both years and no notable change is observed in the channel configuration following the construction and failure of the beaver dams. Would the changes observed at Upper Owens similar to those observed at Boundary if it were not for the posts?

As beaver populations continue to expand it is increasingly important to understand the influence of not only active beaver dams, but also those that fail and are abandoned. Furthermore it is vital that restoration practitioners, working in streams occupied by dam building beavers or those utilizing beavers in their restoration effort, consider beaver dam failures as a part of their expectation management.

Comments

This work made publicly available electronically on December 19, 2012.

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