Date of Award

2016

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Watershed Sciences

First Advisor

Joseph M. Wheaton

Abstract

North American beaver (Castor canadensis) alter stream channel morphology, hydrologic processes, and instream temperature regimes, yet there are few data driven studies that investigate the effect of beaver on stream channel complexity and stream temperature regimes across multiple spatial and temporal scales. The use of beaver as a restoration tool is a method at the forefront of watershed restoration, however little is known about the implications of this restoration technique, particularly with regard to its ability to alter stream channel complexity and stream temperature. This thesis addresses two knowledge gaps with the following objectives: to quantify the role of natural and artificial beaver dams in shaping i) the instream channel complexity and ii) the thermal heterogeneity in a central Oregon stream during summer months.

To address objective i we used high resolution topographic imagery of the stream channel to classify in channel geomorphic units within four paired treatment and control reaches in Bridge Creek. Geomorphic units were analyzed at the treatment and control reach scale. Treatment reaches received the installation of beaver dam support structures, which are designed to either support existing natural beaver dams, or to mimic natural beaver dams, while control reaches lack this restoration treatment, but partially contain natural beaver activity. This study quantified stream channel complexity in terms of: number and diversity of geomorphic units, area and volume of geomorphic units, and spacing of geomorphic units. I demonstrated that reaches under the influence of natural beaver dams or artificial beaver dam structures showed overall increased channel complexity. These reaches showed a greater diversity and number of geomorphic types and larger average area of pools, demonstrating that stream channel complexity was increased in reaches influenced by natural and artificial beaver dams.

To address objective ii we used stream temperature data from 140 temperature loggers deployed in three pairs of beaver influenced and control reaches varying from 90 to 240 meters in length to analyze stream temperature at spatial, temporal, and spatiotemporal scales. The spatial scale results suggest that at all three site pairings, the heterogeneity of stream temperatures is increased in the beaver influenced reaches on the upswing of the diel cycle (at 12:00 PM). At the temporal scale, the data show that stream temperatures are either dampened or slightly lagged in beaver influenced reaches in comparison with the control reaches. At the spatiotemporal scale, no clear pattern in stream temperature heterogeneity emerged in all three site pairings; however the distributions of the stream temperature datasets in beaver influenced reaches differed in comparison to the control reach datasets. A clear pattern of cool water upwelling downstream of two beaver dams in the Lower Owens beaver influenced site was visible in the field and within the dataset at all times throughout the deployment period, though this upwelling was not visible in the other two beaver influenced reaches. The results of this study indicate that stream sections containing natural and artificial beaver dam complexes may provide critical rearing habitat for ESA-listed steelhead trout (Oncorhynchus mykiss) during warm summer months within this semiarid watershed when stream temperatures often exceed lethal limits.

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