Condit Dam Removal Condit Dam Removal - Access Washington

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Condit Dam Hydroelectric Project Final Supplemental EIS salmon). There is some variation in age-at-return for Chinook and chum salmon year-classes and over several generations, the lost year-class would rebuild. The number of generation cycles required would be dependent on the survival rates of year-classes immediately prior and subsequent to the year-class lost. In the case of chum salmon, the number of documented spawning adults in the White Salmon River is very low and likely represents strays from a population below Bonneville Dam that have the potential of eventually recolonizing the White Salmon River basin and reestablishing a viable population. The loss of one or two stray chum salmon during the removal of Condit Dam would not be expected to have a measurable effect on the number of potential chum salmon colonists entering the White Salmon River and other Columbia River tributaries above Bonneville Dam. Sediments released during the initial draining of the reservoir also have the potential to damage or fill-in the intake structure and ponds of the U.S. Fish and Wildlife Service’s fish rearing facility at RM 1.4. PacifiCorp would take measures to protect and restore the rearing facility from high flows and reservoir sediments. During the high flows immediately following the beaching of the dam, the White Salmon River would compose approximately 7 percent of the average flow of the Columbia River (G&G Associates 2004a and 2004b, PacifiCorp 2005). During the period immediately following the breaching of the dam, suspended sediment concentrations would be relatively high. After mixing with the Columbia River for about three miles downstream from the mouth of the White Salmon River, water in the reservoir may have suspended sediment concentrations of 2,300 to 5,800 ppm; at Bonneville Dam, the concentrations may range from 1,100 to 2,600 ppm; and in the lower Columbia River, the concentrations may range from 900 to 2,200 ppm (G&G Associates 2004a and 2004b and PacifiCorp 2005). Except for the immediate vicinity of the mouth of the White Salmon River, Columbia River fish are unlikely to be harmed by sediment entering the river, but they may be displaced from the most sediment-laden portions of the plume until it has completely mixed with the Columbia River, approximately three miles downstream from the mouth of the White Salmon River. Beyond this point, the plume will continue to dissipate through settling and further dilution by Columbia River tributaries, but may briefly interfere with foraging behavior and predatorprey relationships through the Bonneville pool and downstream of Bonneville Dam (PacifiCorp 2005, Korstrom and Birtwell 2006). Predation upon juvenile salmonids will be minimized because dam breaching will occur after the downstream migration of salmonid smolts through the Bonneville pool has been completed, when few juvenile salmonids are present or rearing. After the initial six hours, the White Salmon River would return to a normal flow and make up only approximately 0.6 percent of the Columbia River flow. This would dramatically reduce suspended sediment concentrations as they mix with the Bonneville pool. The anticipated concentrations would range from 200 to 1,700 ppm after complete mixing with the Columbia River a few miles downstream from the mouth of the White Salmon River; from 100 to 750 ppm at Bonneville Dam; and from 80 to 625 ppm in the lower Columbia River (G&G Associates 2004a and 2004b and PacifiCorp 2005). After approximately the first week, sustained concentrations in the Columbia River would be at essentially background levels, with brief spikes in concentrations occurring over the first month. Fish should only be displaced from an area on the north side of the river downstream of the mouth 4.3-19
Condit Dam Hydroelectric Project Final Supplemental EIS of the White Salmon River, and migration in the Columbia River should not be impacted. Downstream of this area, interference with foraging behavior and predator-prey relationships may persist for two to three days and intermittently for several weeks within Bonneville pool. Downstream of Bonneville Dam, no potentially adverse effects are anticipated after the initial sediment plume passes (PacifiCorp 2005). Even after episodic suspended sediment levels drop to sub-lethal levels (probably by six months) fish may exhibit avoidance behavior and stray to nearby watersheds. Sub-lethal levels of average sustained concentrations of suspended sediments would continue for approximately an additional six months, causing sub-lethal (physiological, behavioral, or habitat) effects to fish and other aquatic organisms. Once migration resumes for steelhead trout in the lower river, fish can migrate to less turbid habitat upstream of the lakebed where resting pools are available. If one assumes that no barriers to migration exist at the dam and coffer dam sites in spring and early summer, summer-run steelhead returning to the river in the summer following dam breaching should be able to pass upstream to suitable spawning habitat above the reservoir site. High levels of turbidity during the first six months following dam removal may prevent winter-run steelhead from entering the White Salmon River. These fish would likely seek out other rivers in which to spawn. If White Salmon River winter-run steelhead remain in the Bonneville pool until turbidity levels drop in the spring, they may not be able to pass the cofferdam site until removal is completed. If winter-run steelhead cannot access spawning gravels above RM 5.0 before late spring (May or June), a year-class of age-0 (juveniles produced during the spring of the year of dam removal) winterrun steelhead will be lost. This would substantially reduce the number of expected returning adult winter-run steelhead 4 years in the future, when the majority of the lost year-class would have been expected to return. During that year, the return of winter-run steelhead would be primarily composed of 3-year-old steelhead and strays from other river basins. Returns of winter-run steelhead would likely be reduced every fourth year for several generation cycles. A portion of the previous year-class of steelhead juveniles (age-1 fish) would also be lost. This impact will be long term and recovery will occur in a similar fashion to that of chum and fall-run Chinook salmon. Until fine sediments deposited from the reservoir are cleaned out of gravels by higher velocity storms or spring runoff and the effects of salmon cleaning their redds during spawning, gravels will be embedded and egg-to-fry survival would be poor in the reach of the river from the head of the lake bed (RM 5.0) to the mouth of the river. As levels of suspended sediment are reduced over time and channel-building flow events redistribute bedload through the stream channel, pool habitat would deepen and gravels and cobble will become less embedded. Deposition of fine sediment between gravel particles is a natural process that happens in riverbeds. The spikes in sediment concentration in the White Salmon River described in Section 4.1-4 are related to potential major floods that would provide sufficient movement of bedload sediments to mitigate the plugging of gravels by fine sediment. The predicted spikes in sediment concentration after five years are minor compared to the amount of natural sediment carried in a glacial river draining the Cascade Mountains during flood stage. West coast salmonids evolved in and are adapted to a natural environment that sends periodic pulses of suspended and bedload sediments down river channels. 4.3-20
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Condit Dam Removal Final SEPA Suppl
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March 23, 2007 Dear interested part
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Condit Dam Hydroelectric Project Fi
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Appendix A Memorandum of Agreement
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Appendix C Supplemental Aquatic Res
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Agency and Organization Letters and
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Agency and Organization Letters Con
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Contents Public Meetings Response P
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Contents Individuals Individuals A-
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