Dworshak's Cool Outflows Hit Maximum with
"Hells Canyon is starting to put out more water and that water is warm."
Spill levels were pushed up late Wednesday morning at west-central Idaho's Dworshak Dam in order to flush more cooling water into the lower Snake River ahead of advancing temperatures there.
Hydro system and salmon managers late each summer try to balance Dworshak reservoir outflows between the need to temper rising river temperatures and making the limited water supply last through August and well into September. The goal is to keep the river temperature, as measured at the lower Snake's Lower Granite Dam tailwater, below 68 degrees.
Temperatures higher than that stress migrating salmon and steelhead. Snake River fall chinook salmon and steelhead are listed as threatened under the Endangered Species Act; spawning Snake River sockeye salmon are endangered.
This week, amidst what is typically the Columbia-Snake river basin's hottest period, the managers were forced to push Dworshak's outflows to a maximum of about 14,000 cubic feet per second with roughly 10 kcfs flowing through the dam's three turbine units and 4 kcfs spilled through regulating outlets (ROs) located halfway down the face of the dam.
The 14 kcfs output is the maximum that can be sent downriver without exceeding state water quality guidelines for regulating levels of total dissolved gas, which can also affect the health of salmon and other aquatic organisms.
The coolest available water, about 41 degrees F is tapped by the ROs but the cascading water also stirs up TDG. The turbine units were emitting 43.5 to 44-degree water at midweek to mix with the water from the outlets to flow down the last few miles of the North Fork of the Clearwater River, the Clearwater River and then the Snake. It is estimated that, under current flow conditions, it takes about three days for water to flow from Dworshak down to Lower Granite.
The U.S. Army Corps of Engineers' Dan Feil accurately predicted Wednesday that "we're going to 68 (degrees) today." The Corps operates Dworshak, Lower Granite and seven other dams on salmon's migration path up and down the Columbia and Snake rivers.
The Lower Granite tailwater temperature did rise above 68 degrees for eight hours on Wednesday morning and afternoon, topping out at 68. The temperature then slid back below 68 and has remained in the 67-68 degree range into this morning.
Given the volume water likely to be available, the Corps' Steve Hall told the Technical Management Team that there is roughly a week's worth of 14 kcfs flows, a week of 12 kcfs flows and 10 kcfs flows for the balance of August would put the reservoir near end-of-month elevation targets.
The reservoir at midnight Thursday was at 1,573 elevation, having lost about 1 ½ feet in the first day of the 14 kcfs operation. NOAA Fisheries 2008 Federal Columbia River Power System biological opinion says the reservoir elevation should target 1,535 on Aug. 31, and draw down to 1,520 (80 feet below full pool) by the end of September. The BiOp prescribes measures that NOAA Fisheries feels are necessary to avoid jeopardizing listed salmon and steelhead. The TMT's federal, state and tribal membership helps guide day to day hydro operations for the benefit of fish.
"We'll back off when we can," Feil said of the desire to reduce Dworshak outflows as soon possible to reserve water for later in the season. The TMT was scheduled to meet again today to reassess the situation.
The situation has been made more difficult by a heat wave that is forecast to continue into next week and by higher than normal flows out of the Hells Canyon Complex of dams upstream of Lower Granite and the Snake's confluence with the Clearwater. Water temperatures measured at Anatone, located downstream of the complex, rose well above 74 degrees Thursday.
In recent days Hells Canyon outflows have bounced up and down from lows of 12 kcfs to highs of nearly 23 kcfs.
"When they take it up to 22-23 that's pretty hard to counteract" with flows from Dworshak, Hall said.
"Hells Canyon is starting to put out more water and that water is warm," Feil said. "It's going to exacerbate the situation." Clearwater temperatures were even higher late this week as measured at Orofino, Idaho, which is located above the river's confluence with the North Fork.
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