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NW Fishletter #373 September 5, 2017

[3] Cool Water Still Needed From Dworshak as Fish Continue Migration

With inflows to Dworshak Dam's reservoir decreasing--and no significant precipitation in sight--dam operators began reducing outflows in August both over the spillway and through generators.

Yet salmon and steelhead migrating in the lower Snake River still need cool water, and so far flows released from Dworshak have kept water temperatures at Lower Granite and Little Goose dams on the lower Snake River mostly below 70 degrees during the June 21-Aug. 31 summer fish passage season.

"We can't continue to release 11 kcfs through September because the reservoir would be drafted too low," Steve Hall told TMT members Aug. 9.

Cold water drawn from deep in the reservoir and pushed over the spillway helps lower river temperatures downstream of Dworshak. Water temperatures exceeding 68 degrees can stymie adult salmon and steelhead migrating upstream to spawning areas.

The 11-kcfs discharges--more than half of it spill--from Dworshak Dam helped lower water temperatures despite the soaring 100-degree-plus air temperatures in this area near Lewiston, Idaho.

Even though water temperatures downstream of Dworshak at Lower Granite and Little Goose dams have hovered around 69, they have been lower than temperatures farther downstream at Lower Monumental and Ice Harbor dams, the other two dams on the lower Snake River.

U.S. Fish and Wildlife Service and Nez Perce Tribe co-manage the hatchery. Credit: Dworshak Hatchery.

Hall proposed to drop to 10 kcfs in mid August; then when temperatures at Lower Granite Dam fall to 68, drop water discharge to 9 kcfs.

While the cooler water from Dworshak spill benefits migrating adult and juvenile fish, total dissolved gas levels produced by the spill can be a hazard to fish in Dworshak National Fish Hatchery downstream of the dam.

The decrease in spill will reduce total dissolved gas levels at the propagation facility. In the hatchery setting, young salmon and steelhead are vulnerable to gas bubble disease because they are unable to dive deep in the hatchery's shallow water to avoid the gases that accumulate in the top of the water column.

This year's elevated TDG levels are a result of a higher outflow ratio of spill to generation because Dworshak's largest generator has been off line since September 2016, making a challenging situation for salmon and for dam managers trying to cool lower Snake River water while keeping total dissolved gas levels low.

Because Dworkshak Dam blocks upstream and downstream fish passage, the dam's spill does not aid juvenile salmon and steelhead by sweeping them over or around the dam generators as happens at eight of 31 Federal Columbia River Power System dams. Juvenile fish are, however, helped by the cooler water temperatures.

TMT members did not object to the change in operations at Dworshak Dam. Russ Keifer, representative for Idaho Department of Fish and Game, reported that virtually all the sockeye headed to the Salmon River basin had now passed Lower Granite and were starting to show up in the valley. As of Aug. 28, 227 Snake River sockeye were counted at the dam.

Dave Swank, a U.S. Fish and Wildlife Service biologist and representative of the hatchery, said he welcomed the relief from lower discharges. Swank said tests at the hatchery showed some fish with bubbles in their gills, but there hadn't been elevated mortalities and all the young fish were eating normally.

At an earlier TMT meeting, Swank indicated TDG levels in the hatchery had reached as high as 104-105 percent.

TDG in the lower Snake has been at 115 percent and lower in recent weeks. According to a Clean Water Act waiver approved by the state of Idaho in conjunction with the Nez Perce Tribe, TDG mustn't exceed 121 percent. -Laura Berg

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: Laura Berg
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