| Summary: | 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 rivers 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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