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Study Looks at Benefits of Dworkshak Flows on Adult Fish

by Mike O'Bryant
Columbia Basin Bulletin - January 24, 2003

Cool water releases from Dworshak Dam in September 2002 may have yielded a few more steelhead and fish that passed the lower Snake River dams also had lower body temperatures, according to Chris Peery of the University of Idaho.

A report by Peery on the effects of augmenting Snake River flows with water from Dworshak Dam in the Clearwater River basin during the first 10 days of September is not conclusive on whether the practice yields significantly more fish. However, it also found that travel time for adults during the period remained the same as when flows were not augmented, but that the average body temperature of steelhead that pass through the four dams is lower by up to 4 degrees centigrade than fish that passed the dams during the 2001 low water and that could contribute to better survival.

In most years, the Technical Management Team releases water from Dworshak Reservoir in July and August to help cool the lower Snake River to aid juvenile and adult salmon passage. However, that water typically gets used up by the end of August. Seeing a higher than normal snowpack in the Clearwater River basin in 2002, CRITFC, the Nez Perce Tribe and the Idaho Department of Fish and Game asked TMT to extend cool water flows from the dam through Sept. 10. That gave Peery the opportunity to study the effects on adult steelhead of extending the flows.

"The more fish that get out of the reservoir earlier, the further they will penetrate the tributaries," Steve Pettit, IDFG, said about one way the cooler flows help adult fish. "That makes a big difference."

Peery said the water from Dworshak has the potential to cool lower Snake River flows, but it doesn't necessarily have a cooling effect on every level of the river. He said that the cool water from Dworshak stays in the lower strata of the Lower Granite Reservoir and shows up cooling the tailwater at the dam. Water in the forebay, although showing some small effects from the cooling, is largely influenced by weather, Peery said.

In addition, water flowing from the reservoir through the fish ladder tended to be warmer than further down on the fish ladder where water is injected from the tailrace.

He found that there was a slight increase in the number of fish in the river in early September when comparing the years 2000 and 2002 (2001 was a drought year), but the most he would say of the results is that having the cool water in the system did not prohibit the fish from moving.

Pettit said past experience showed that when the "spigot was turned off, there was a significant change in fish movement until the fish got used to the water temperature."

TMT adopted the operation last year because there was enough water to provide the 200 acre feet needed, but the way this year's snow pack and water supply are shaping up could prohibit a second year for the early September flows.

"We have to weigh when to provide the water," said Rudd Turner of the U.S. Army Corps of Engineers. "Last year there was enough water, but last year there was about twice the amount of water than what's predicted this year. We may not have the luxury."

Related Sites:
Technical Management Team:

Mike O'Bryant
Study Looks at Benefits of Dworkshak Flows on Adult Fish
Columbia Basin Bulletin, January 24, 2003

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