Year of Award

2014

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Andrew Wilcox

Commitee Members

Johnnie Moore, Lisa Eby

Keywords

disturbance, transport capacity, sediment pulse, grain and bedform roughness

Abstract

The Condit Dam breach on the White Salmon River (WSR) in Washington provided a unique opportunity to study how a bedrock-confined, gravel-bed river responds to a large influx of fine reservoir sediment. On October 26, 2011, a dynamite explosion breached a hole in the base of the 38 m tall dam, causing rapid reservoir erosion and downstream transport of fine sediment through the 5,300 m of channel separating the reservoir from the mouth of the WSR, where it flows into the Columbia River. In my research, I combined field data, aerial photographs, and LiDAR surveys to measure pre-breach and post-breach geomorphic conditions, up to 9 months after the breach, to assess downstream geomorphic response through a confined reach (reach 1) with forced pool-riffle morphology and a less-confined reach (reach 2) near the river’s mouth. I found that the magnitude and duration of geomorphic adjustment was smaller over riffles than pools and over reach 1 than reach 2. By 3 weeks after the dam breach, pools stored about twice as much of the reservoir-derived sediment (~95,000 cubic m) as riffles (~50,000 cubic m). By 9 months post-breach, nearly all (90%) of the sediment had been evacuated from riffles (~5,000 cubic m remained), whereas about half of the sediment initially stored in pools had been evacuated (~50,000 cubic m remained). Reach 1 stored ~145,000 cubic m within the 3 weeks after the dam breach compared to the 650,000 cubic m stored in reach 2. By 9 months post-breach, the volume of sediment stored in reach 1 (~40,000 cubic m) decreased by 72% and the volume in reach 2 (~490,000 cubic m) decreased by only 25%. I also found significant storage behind large wood deposits and throughout the transition between reach 1 and reach 2. My findings suggest a conceptual model by which reductions in grain and bedform roughness caused by initial sediment deposition in reach 1 contribute to sediment transport and deposition in reach 2. Findings from the WSR can help inform recovery from other sediment disturbances and dam removals.

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© Copyright 2014 Erika J. Colaiacomo