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Corps Moves Forward On Multi-Million Dollar Dalles Dam Juvenile Fish Passage Project
Posted on Friday, May 16, 2008 (PST)

Federal, state and tribal officials have yet to prioritize which research and construction projects will be funded in the fiscal year 2009 budget for the Columbia River Fish Mitigation program, but they'll have to fit their choices around construction of a second spillwall below The Dalles Dam.

Preliminary estimates are that the 850-foot-long spillwall will cost $19 million in 2009 and a like amount in 2010 to complete construction. By providing migrating juvenile fish a direct route to the deepest, fastest channel of the river, it is estimated that the spillwall will improve dam-passage survival by 4 percent for yearling chinook salmon and steelhead and 3 percent for subyearling fall chinook, according to an overview of the U.S. Army Corps of Engineers project.

"The contract's out on the street," Corps project manager Lance Helwig said of the call for bids. The Corps, which operates the dams, plans to review the bids received and likely choose the winner in mid-July.

Construction would begin in the fall and likely extend through an Oct. 1 through April 10 work window in both years. The normal winter work window is Dec. 1-Feb. 28 but more time is needed because the spill wall is such a large project and involves so-much in-water work, Helwig said. Such work is planned in midwinter when the fewest number of migrating salmonids are in the river.

Spillway survival has long been an issue at the dam, and roughly 80 percent of the outmigrants use that route of passage. The rest go through the turbines or its trash sluiceway, the latter having the highest survival rate of the three passage routes. The Dalles Dam has no mechanical juvenile passage system as do the other dams in the federal Columbia/Snake hydro system.

A 100-foot spillwall between spillbays 6 and 7 was completed in 2004 in an attempt to reduce direct mortality and predation, primarily by northern pikeminnow, by disrupting a lateral flow below the spillbays that seemed to serve the salmon up for waiting predators.

The sideways flow seemed to batter fish, injuring and/or killing many.

"At the same time, they'd get tired, fatigued," which made them more susceptible to predators, Helwig said.

Overall spillway mortality, percentage-wise, could be at times in the high teens, according to the Corps' Mike Langeslay.

The wall "broke up that lateral component" and resulted in reduced direct mortality and injury. The predation issue, however, just changed complexion. The altered flow resulted in some of the fish passing over a shallow spillway shelf where avian and fish predators awaited.

The egress route also passed the Bridge Islands a short distance below, where shallow, slow currents also gave predators an edge.

Langeslay said the current dam survival scenario was not worsened, but the continuing predation reduced the anticipated benefit from the 100-foot wall.

The planned wall is expected to provide fish quicker egress and a direct conveyance from the spillbays to the thalweg, the main river channel where the young salmon have a chance of outswimming the pikeminnow.

The new spillwall will jut straight downriver from, roughly, the middle of the 20-spillbay spillway. It will have a slight bend at the end, shaped somewhat like a hockey stick. The wall will be 10 feet wide and 43 feet high for its first 200 feet and 25 to 30 feet high for the remainder of its length.

Engineers had considered an extension of the existing spillwall to the thalweg. But modeling showed the extension would likely cause an unacceptable increase in total dissolved gas, which can be harmful to fish, while the proposed new structure would not significantly change TDG.

"We went through a pretty rigorous numerical analysis of that" last summer before deciding to build the new wall, Helwig said. The strategy received the blessing of regional entities involved in the process. In a CRFM program rating system used to prioritize projects, the new spillway received a score of 4.9 on a scale of 5 in voting by representatives of federal, state and tribal fish and wildlife and hydro entities.

The proposal was reviewed in technical committees that included representatives of the Corps, NOAA Fisheries Service, the Bonneville Power Administration, the U.S. Fish and Wildlife Service, Idaho, Oregon and Washington fish management entities, the Columbia River Inter-Tribal Fish Commission and the Northwest Power and Conservation Council.

Proposals for CRFM program funding are prioritized by the System Configuration Team, which has that same membership.

The overall survival rate is also expected to be higher with the new wall, as the wall location will allow spill to be directed, even during high springtime flows when more of the spillbays must be opened.

The new wall, and a spill pattern to be developed over the next two years, will help move the young fish quickly and directly out of the spillway to deep water and help them avoid the shallow south side of the river Bridge Islands

Langeslay said a goal is to lift spillway survival to 98 percent. Under the current configuration, as an example, spillway survival for spring chinook was 92.4 percent.

"We believe we can," Langeslay said of what would be one of the single biggest survival gains still on the table within the Federal Columbia River Power System. Capital improvements at the dams over the past 20 years have slowly improved survival to the point that additional gains are hard to get.

A new NOAA Fisheries Service biological opinion for the FCRPS calls for hydro system improvements aimed at achieving specific performance standards -- 96 percent per dam passage survival for spring juveniles and 93 percent per dam passage survival for summer juvenile migrants averaged across the Columbia or Snake river dams by the end of a ten-year period. The spillwall is one of the actions called for in the BiOp, in time for the 2010 outmigration.

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