Effects of Dam Removal on Tule Fall Chinook salmon Spawning Habitat in the White Salmon River, Washington

Abstract

Condit Dam is one of the largest hydroelectric dams ever removed in the USA. Breached in a single explosive event in October 2011, hundredsof-thousands of cubic metres of sediment washed down the White Salmon River onto spawning grounds of a threatened species, Columbia River tule fall Chinook salmon Oncorhynchus tshawytscha. We investigated over a 3-year period (2010)(2011)(2012) how dam breaching affected channel morphology, river hydraulics, sediment composition and tule fall Chinook salmon (hereafter 'tule salmon') spawning habitat in the lower 1.7 km of the White Salmon River (project area). As expected, dam breaching dramatically affected channel morphology and spawning habitat due to a large load of sediment released from Northwestern Lake. Forty-two per cent of the project area that was previously covered in water was converted into islands or new shoreline, while a large pool near the mouth filled with sediments and a delta formed at the mouth. A two-dimensional hydrodynamic model revealed that pool area decreased 68.7% in the project area, while glides and riffles increased 659% and 530%, respectively. A spatially explicit habitat model found the mean probability of spawning habitat increased 46.2% after dam breaching due to an increase in glides and riffles. Shifting channels and bank instability continue to negatively affect some spawning habitat as sediments continue to wash downstream from former Northwestern Lake, but 300 m of new spawning habitat (river kilometre 0.6 to 0.9) that formed immediately postbreach has persisted into 2015. Less than 10% of tule salmon have spawned upstream of the former dam site to date, but the run sizes appear healthy and stable. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Coastal habitat and biological community response to dam removal on the Elwha River

“…Downstream fish populations often benefit from dam removal because overall habitat conditions are improved (Hatten et al. ), but long‐term declines can also occur (Burroughs et al. ).…”

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“…The sediment filled pools throughout the lower river created gravel bars and a small delta at the Columbia River confluence ( Figure 6). In 2014, the lowermost 0.5 km of the White Salmon River was still influenced by Bonneville Pool but was only 2 m deep (Hatten et al 2015). As of 2015, the river had not had a large flood event since dam removal, and the sediment upstream and downstream of the project area is expected to change.…”

“…The reaches downstream of Condit Dam average approximately 1% in gradient. Prior to the release of sediment behind the dam, the gradient of the lowermost 1.6 km of the White Salmon River was influenced by the backwater effect of the Columbia River's Bonneville Pool (impounded by Bonneville Dam), and the water in the lowermost 0.8 km was more than 9 m deep ( Figure 6; Hatten et al 2015). The outflow of silt, sand, and gravel from dam breaching aggraded the lower river by about 1.5 m (Wilcox et al 2014).…”

“…After the final blast, the 2.9-km-long reservoir (1.6 million m 3 of water) drained in less than 2 h and released 10% of the 1.8 million m 3 of impounded sediment (Figures 4 and 5). This was the largest volume of sediment ever released in the United States by breaching a dam with explosives (Hatten et al 2015).Video of the breach and time-lapse footage of the draining of the reservoir can be viewed at: video.nationalgeographic.com/video/news/us-condit-dam-breach-vin. Within eight weeks, about 60% of the reservoir sediment had exited through the tunnel (Wilcox et al 2014).…”

“…Fall Chinook Salmon redds were mapped in coordination with a two-year bathymetry survey of the effects of dam removal on Tule fall Chinook Salmon spawning habitat conducted by USGS (Hatten et al 2015). The USFWS surveyed for juvenile lamprey presence in the lower river postremoval.…”