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NW Fishletter #379, March 5, 2018

[4] High Snowpack, Off Line Generator Mean More Spill At Dworshak Dam

The U.S. Army Corps of Engineers agreed Feb. 28 to reduce discharge flows at Dworshak Dam to 16,000 cubic feet per second through March, barring unforeseen changes in runoff predictions or weather causing a sudden spike in the amount of water flowing into Dworshak Reservoir.

The plan was one of three options for operating the dam's outflows this spring, presented to the Technical Management Team by Steve Hall, the Corps' Walla Walla District reservoir manager. Other options included gradually stepping down outflows to 14,000 cfs over the first week of March, or keeping the outflow at 20,000 cfs for another week and then dropping them to 14,000 cfs.

Dworshak Dam discharge was boosted to about 20,000 cfs on Feb. 16 to make room for more water in the reservoir in a spring that, by all forecasts, looks to be above normal for runoff. A recent forecast for water supply in the Clearwater River basin shows runoff at 124 percent of normal, Hall told the TMT. "That's well above average and well above recent years on record," he said. "And we're continuing to build snow at a pretty rapid pace."

Current forecasts call for a total runoff of about 3 million acre-feet, Hall said. So far, this water year is tracking with 2011--the last time the Clearwater basin saw a huge runoff of about 4 MAF. "We don't think inflows [this year] will be that strong, but it's distinctly possible," he said. "But we would have to have a lot more snow accumulate" to reach the 2011 levels.

Dworshak Dam. Courtesy USACE

The Corps must release water from the reservoir now to prevent local flooding and flooding downstream in the Snake River and Columbia River system, as snow begins to melt this spring. "From a flood control perspective, we think making a little bit more space sooner rather than later is a good idea, but relieving the total dissolved gas is something we see as a needful action as well," Hall said.

Handling this much runoff is not usually a problem at Dworshak, John Heitstu-man, Walla Walla District's chief hydrologist, told NW Fishletter. But with Unit 3, the dam's largest generator, off line since September 2016, only about 5,000 cfs instead of 10,000 cfs can run through the powerhouse, leaving the remainder to spill over the top of the dam. That creates higher total dissolved gas levels in the river below, and at Dworshak National Fish Hatchery, which uses Clearwater River water in its hatchery operations. The hatchery is co-managed by the U.S. Fish and Wildlife Service and the Nez Perce Tribe of Idaho.

A second hatchery, the Clearwater Fish Hatchery, is managed by the Idaho Department of Fish and Game, but water supply for that hatchery does not come from the river, so it's less impacted by the higher TDG levels, according to hatchery supervisor Beau Gunter.

High TDG levels stress fish, and can reduce survival rates. The State of Idaho limits TDG to 110 percent, but dam operators can exceed that limit when needed for flood control.

On Feb. 28, before the Corps reduced the spill over Dworshak, dissolved gas was about 122 percent in the river, and 104 percent at Dworshak hatchery, where degassing units help reduce those levels.

Jay Hesse, TMT member and director of research at the Nez Perce Tribe of Idaho's fisheries department, told NW Fishletter the excess spill last spring resulted in trauma to fish at the Dworshak hatchery, which uses water from the river. "We didn't see mortality at unexpected levels in the hatchery, but we did see the gas bubble trauma symptoms, and that can have a cumulative effect on survival later in life," he said. Because of the TDG levels and stress on juveniles, the hatchery re-leased many of its juvenile fish about two weeks early and in different locations last spring.

Hesse said that those juvenile fish survived at normal levels when they reached Lower Granite Dam. More recent data shows that last year's juveniles throughout the system had very low survival rates through the lower Columbia River system, and it's unknown whether higher dissolved gas played a role in those low numbers. He added that the real test will come in two to three years, when they can track how many of these hatchery fish return.

The hatchery is now discussing the possibility of releasing juveniles one or two weeks early again this year, but that decision hadn't yet been made, David Swank, TMT member from the USFWS told fellow TMT members. He said immediately dropping outflows to 16,000 cfs would give the hatchery fish, especially steelhead, a respite before they're released.

Gunter said the Clearwater hatchery's main concern last year was that they'd be releasing their young fish into the river when TDG levels were high, but dam operators were able to reduce spill for three days, providing them a window for releasing into better conditions.

"We have survival estimates for that group, and there didn't appear to be anything that was ab-normal," he said. After last year's experiences, Gunter said "It seems to be going a little smoother. We just hope they can get the turbine in and these problems don't continue for another year."

Hall told the TMT contractors are making "good progress" toward fixing the Unit 3 turbine, and expect to finish sometime in June. -K.C. Mehaffey

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NW Fishletter is produced by Energy NewsData.
Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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