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NW Fishletter #394, June 3, 2019

[4] Corps Proposes Underwater Construction At Detroit Dam

The U.S. Army Corps of Engineers is now proposing underwater construction instead of a one- to two-year drawdown to build a water temperature control tower and fish collection structure at Detroit Dam on the North Santiam River southeast of Salem, Ore.

The agency on May 24 released its draft environmental impact statement (EIS) for the Detroit Downstream Fish Passage Project, backing off proposals to draft the reservoir well below normal operations after the analysis found a two-year drawdown would cost the region's economy more than $200 million.

"When we started scoping a year and a half ago, we hadn't realized all the impacts to downstream water users," Jeff Ament, the Corps' project manager, said in a news release.

Some of those economic impacts were outlined in U.S. District Court filings in Northwest Environmental Defense Center et al. v. U.S. Army Corps of Engineers et al.

Marion County and the City of Salem say in a cross claim that nearly 800 companies with more than 16,000 employees rely on water from the lake, mostly for agriculture and food processing. Many industries would likely relocate without a permanent, stable water supply, the filing states. Salem, which relies on Detroit Lake for its drinking water, contends that a drawdown would also interfere with its water rights and threaten human health and safety.

The draft EIS, available for public review and comment until July 23, analyzes five alternatives, including a no action option. Four other alternatives include constructing a 300-foot temperature control tower attached to the dam that would enable the Corps to mix warm surface water with the cooler water from deep below the surface in order to meet temperature targets downstream.

The preferred alternative--to build the temperature control tower underwater, using divers and operating from floating barges--would cost an estimated $200 million to $250 million. An alternative that would drain the reservoir to 1,300 feet for two years and involve no underwater construction would cost between $100 million and $200 million.

All four action alternatives also include a downstream passage structure at an estimated cost of $250 million to collect juvenile Chinook and steelhead in the reservoir to be hauled and released downstream. An existing adult collection facility would continue to be used to provide upstream passage.

The Corps also analyzed numerous other alternatives--21 for temperature control, eight for fish collection and three for fish transport--that were eliminated for a variety of reasons. Many of the rejected alternatives were not technically feasible, posed risks to dam safety or the Corps' ability to manage floods, or would eliminate purposes of Detroit Dam over time.

The preferred alternative would have the lowest impact, according to the draft EIS. "Under the preferred alternative, there would be short-term, localized impacts to air quality, noise, sediment transport, turbidity, fish and wildlife, and vegetation," it said. Moderate impacts to resident fish and kokanee are expected from underwater blasting.

"Improving downstream fish passage and temperature control at high-head dams is extremely complex and challenging," the draft EIS said. Because it is operated for flood risk management, there are large seasonal fluctuations in reservoir elevation. But the project is operated for several other purposes, including water supply, hydropower, water quality, fish and wildlife and recreation. With two generating units, the dam has a capacity to generate 100 megawatts of power.

"Balancing these missions while also moving fish around a 400 foot barrier in a reservoir that moves up and down more than a hundred feet over the course of every year is very challenging from a technical and biological standpoint," the dEIS said. "Therefore, the Corps spent many years developing, assessing, and screening numerous alternatives for both downstream passage and temperature control."

Downstream fish passage and temperature control are required under a 2008 biological opinion to avoid jeopardizing upper Willamette River Chinook salmon and steelhead, both listed as threatened under the Endangered Species Act. In summer, water released from the dam is too cold, causing some adults to stop migrating and spawn downstream. In the fall, water released from the dam is too warm, causing eggs downstream to die or hatch too early.

The temperature control tower--also called a selective withdrawal structure--would have sliding high intake weirs to move warm surface water, and low intake gates to move cool water from below the surface, through the dam, depending on the needs at the time.

The juvenile fish collection facility would be a floating screen structure attached to the temperature control tower to collect migrating juvenile fish. The 300-foot by 100-foot structure is designed to capture juvenile fish from reservoir outflows and hold them until they are transported downstream.

The proposal would improve survival rates of juvenile Chinook and steelhead, and survival of returning adult fish migrating up the North Santiam River to spawn, the draft EIS said. A final EIS and record of decision are expected next year, followed by three years of construction of the temperature control tower, and then three years of constructing the fish collection facility.

The Corps is also working to develop a new biological opinion and a new EIS for all 13 projects in the Willamette Valley System, while continuing with efforts to resolve downstream fish passage issues at Detroit and Cougar dams. -K.C. Mehaffey

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NW Fishletter is produced by NewsData LLC.
Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
Phone: (206) 285-4848 Fax: (206) 281-8035

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