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NW Fishletter #367, March 6, 2017

[4] Balancing Act at Dworshak More Acute with Turbine Out of Service

With Dworshak Dam's largest turbine out of service for a major overhaul, dam operators and salmon managers have been kept on their toes juggling flood-control requirements, total-dissolved-gas limits, and refill and flow-augmentation needs.

The Unit 3 turbine-generator, which has a capacity of about 220 MW, has been idle} since September. Unit 1 was also out of commission for several weeks in January for testing.

The outages limit power generation and the amount of water discharged from the dam, and excess water must be spilled. Yet the spill's level of total dissolved gas (TDG) must be kept within Idaho's water-quality limit of 110 percent.

Spilling increases the amount of gas present in the water, which can hurt fish, although moderate levels of spill help juveniles on their downstream journey to the ocean.

Dworshak's operator, the Walla Walla District of the U.S. Army Corps of Engineers, and the interagency Technical Management Team (TMT) are meeting weekly to balance flood-control requirements with the needs of fish and the hydroelectric system.

Unit 3 repairs are two months behind schedule, pushing forward the challenges further into the juvenile fish-passage season, which is March through early August. The Corps predicts the contractor will complete the overhaul in early August.

With Unit 1 now back in operation, the volume of water releases increased in late February to about 9 kcfs -- divided about 4.6 kcfs for generation and the rest for spill.

Dworshak Dam
Plunging water, here at Dworshak Dam, traps air bubbles in the water. Credit: USACE.

Under the mid to late Feb. spill operation, the reservoir was being drafted 5 feet below its required flood-control elevation to make sure that elevation level was attained. Since early March, higher inflows have increased spill volume just over 120 percent TDG, up from previous levels closer to 110 percent TDG producing 108-percent TDG.

At recent TMT meetings, Steve Hall, the Corps' district reservoir manager, has said the prudent course of action was to move the water early.

Dave Statler, Nez Perce tribal representative to the TMT, asked about the risk of releasing too much water if inflows turn out to be less than anticipated, jeopardizing refill and augmenting summer flows with cooler water.

TMT salmon-manager representatives from Oregon and the Umatilla Tribe said they too questioned the decision to draft water below flood-control levels. The concern is not only for spring-flow augmentation, but also for adequate water supply to release cool water during the summer months.

The February water-supply forecast at Dworshak Dam was 104 percent above average, but the March prediction is 118 percent above average. Both are higher than January's.

Russ Keifer, Idaho's TMT representative, said the decisions made by federal action agencies and salmon managers -- who are all members of the TMT -- create risks to refill and TDG. Idaho's position, he said, is that the greatest risk to fish is from TDG rather than from the loss of augmented spring flows.

The Corps' Hall noted that spring augmentation is very limited now with Unit 3 down.

Keifer and Hall concurred that the big concern is TDG levels in April, when in-flow levels are high and hundreds of thousands of juvenile fish are in the Clearwater and North Fork Clearwater rivers.

According to Idaho Department of Fish and Game and Nez Perce Tribal Fisheries, 50 percent or more of the wild fall Chinook run are expected to emerge from spawning gravel between March 15 and April 1, with fish closest to the dam experiencing the highest levels of TDG.

On March 27, hatchery managers will release 5.5 million spring Chinook smolts into the North Fork Clearwater River, the Nez Perce's Jay Hesse told TMT members. They will need protection from high gas levels, he said.

Steelhead releases from the hatchery are planned to begin April 10 on the main-stem Clearwater. "Levels of 115 percent TDG saturation would probably be acceptable for these fish, but higher levels should be avoided," Hesse said.

Currently the juvenile fish now at the Clearwater National Fish Hatchery, which is just down-stream of the dam, are not in danger because TDG levels are low, and even if levels should rise, the hatchery is equipped with degassers that would take care of the problem.

Hesse described a two- to three-week window in early March before hatchery fish are released when higher TDG levels would have the least impact on fish.

"Prolonged early spill is the 'best of no good options,'" he said. -Laura Berg

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NW Fishletter is produced by Energy NewsData.
Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: Laura Berg
Phone: (206) 285-4848 Fax: (206) 281-8035

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