Condit Dam Removal Condit Dam Removal - Access Washington

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Condit Dam Hydroelectric Project Final Supplemental EIS by Squier Associates (1998), immediately after the final blast to breach the tunnel in the dam, the pH of the stream may reach lethal levels (pH of about 11.4) quickly and then decrease to nominal levels. A check of the assumptions used by Squier Associates and recalculation revealed that any spike in pH is likely to be diluted to less than lethal levels in less than a minute and be near normal in 15 minutes or less. Elevated pH water is not likely to go downriver as far as the powerhouse. Any fish in the vicinity of blasting in-water prior to breaching, especially in the reservoir, would likely suffer hydrostatic shock and die from a ruptured air bladder. A reasonable assumption, based on projected levels of total suspended solids in the water, is that all fish within the White Salmon stream channel below the dam are likely to be killed by the load of suspended solids that would occur directly following dam breaching. Some fish may survive the sediment and be flushed into the Bonneville pool of the Columbia River. Many of the fish in the reservoir would be flushed downstream at this time and most of these fish probably would be killed. Fish that are not flushed below the dam after the initial breaching would either be stranded on the dewatered reservoir substrate or end up in the new stream channel created in the bed of the reservoir. The load of suspended solids would likely kill most of these fish, but some may survive to migrate downstream to the Bonneville pool or upstream into tributaries or the river above the reservoir. All of the benthic macroinvertebrates present in the stream channel, such as freshwater mussels and aquatic insects, also are likely to be killed by suffocation or burial. The level of suspended sediments would diminish over time. Within 7 to 30 days, the average sustained levels of suspended sediments should drop to concentrations that are not immediately lethal (with an exposure of 12–24 hours), but still lethal over an exposure period of several days (Bash et al. 2001). However, where populations of aquatic organisms are eliminated, they will have to recolonize. The length of time needed for recolonization will depend on how rapidly the substrate returns to habitable condition for the species and the distance, accessibility, and mechanisms the colonizing organisms must overcome. Full recolonization is likely to take several years and possibly longer for some. However, there should be more accessible habitat for all of the species except those adapted for reservoir existence. Potential impacts related to petroleum products and erosion in upland areas would be similar to those described above for pre-dam removal activities. Once space is available, a trash rack would be installed on the upstream side of the tunnel to collect woody debris that might plug the tunnel and interfere with sediment transport and fish passage (PacifiCorp 2004). As the tunnel is constructed, holes would be drilled in the sides of the tunnel to provide resting pockets for fish as they pass through the tunnel. This is intended to reduce the velocity barrier to migrating salmonid passage until the dam is completely removed. Sediment Transport Immediately after draining the reservoir, sediment would be deposited downstream in the floodplain areas as the river flow subsides. This sediment would then be further transported downstream during natural storm and flood events and additional sediment from the reservoir and upstream would be moved downstream by the river, especially during storm flows and 4.3-17
Condit Dam Hydroelectric Project Final Supplemental EIS when reservoir sediment slumps occur. The repeated movement of high concentrations of sediment in the water would have major effects on fish and other aquatic organisms for several months. Increases in sediment would prevent fish from entering the river until the concentration of suspended sediments reduces to a level that fish can tolerate during upstream migration (at least the first three months). Fish remaining in the river channel that develops in the bed of the reservoir or chum salmon and winter steelhead migrating through the Bonneville pool may attempt to enter or migrate through the lower White Salmon River after the initial breaching of Condit Dam. Temporary reductions in suspended sediment concentrations may occur during dry periods with low flows, but if fish move into the river, they would be vulnerable to the next spike of sediment concentration. The only refuge habitat would be in accessible reaches of Mill Creek, Buck Creek, and the White Salmon River and its tributaries between RM 5.0 and RM 16.2. These areas would not become accessible to fish moving upriver from below the dam until the removal of the cofferdam in May of the following year. Spawning or rearing habitat would therefore not be available for anadromous salmonids until the removal of the cofferdam. It is reasonable to assume that no natural recruitment of fallrun Chinook salmon or chum salmon would occur during the fall and winter following the breaching of the dam and that one year-class of fry naturally produced in the White Salmon River would be lost. Chum salmon are not likely to enter Bonneville pool or its tributaries until after Condit Dam is breached. The high levels of suspended sediments in the White Salmon River below the dam site are expected to prevent chum salmon spawners from entering, and any chum salmon that may have been expected to spawn in the White Salmon River will either spawn in other tributaries of the Columbia River or drop back over Bonneville Dam and spawn in the Columbia River mainstem. No chum salmon smolts would be produced in the White Salmon River and be imprinted to return to the White Salmon River during the years following dam removal. Hence, a potential year-class of chum salmon smolts imprinted to return to the White Salmon River would not be produced. The number of adult chum salmon documented in the White Salmon River in recent years is very small and is likely composed of strays from a spawning population in the mainstem of the Columbia River below Bonneville Dam. Stray chum salmon from the spawning population in the Columbia River below Bonneville Dam have the potential of providing seed stock for the eventual recolonization of the White Salmon River by chum salmon, but do not represent a viable White Salmon River population. Although natural spawning of fall-run Chinook would not occur during the year of dam removal, PacifiCorp has proposed to capture and transport to a hatchery the fall Chinook returning to the White Salmon River before the dam is breached in October. Out-planting of the juveniles back to the White Salmon River after river conditions are again suitable would have a high chance of preventing a major effect on the year-class. As a result, although a year-class of naturally spawned fall-run Chinook salmon would be lost, the year-class would be supplemented by hatchery smolts produced from spawning Chinook salmon captured from the White Salmon River. Run size during the years the lost year-class would be expected to return as mature spawners would be reduced and composed entirely of spawners from other year-classes. This impact would be long-term (at least several 3- to 5-year generation cycles for Chinook and chum 4.3-18
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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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