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Dam Managers Tweak Reservoir To Aid Fish

by Bill Rudolph
NW Fishletter, August 18, 2009

Water temperatures in Northwest rivers rose along with surface temperatures late last month, causing fish managers on both sides of the border to worry about the heat's effects on both young and old migrating salmon.

But on Canada's undammed Fraser River, where water temperatures are expected to reach nearly 72 F by Aug. 5, managers could only cross their fingers and hope for the best as early-run sockeye migrated upstream.

According to the Pacific Salmon Commission, if the river hit the estimate, it would be the highest Fraser temperature recorded in a nearly 100-year data set. It didn't quite make it, but peaked at 70 F.

In the Columbia Basin, dam operators and fish managers met in late July to see what they could do to improve conditions for migrating fish, both the late-migrating juveniles and returning salmonids.

They agreed to bump up outflows from Idaho's Dworshak Reservoir to 14 kcfs from 12 kcfs to help cool the Snake River above Lower Granite Dam where tailrace temps were slightly below 68 degrees by July 29. Sixty-eight degrees is generally considered a threshold for adverse effects on salmonids and is mandated in the Clean Water Act.

However, the 43-degree water from Dworshak was providing only limited help for fish, because 74-degree water was rolling down the Snake from Brownlee where Idaho Power was faced with mandatory releases from an unusually wet June.

But operational constraints at Dworshak (dissolved gas limits from spill) kept outflows to 14 kcfs, said dam operators at the July 29 meeting of the Technical Management Team. All were on board with the change, acknowledging that by using up more Dworshak water now, less will be available later in the season, when the adult chinook migration will be in full swing.

Managers were hoping that water in the tailrace at Lower Granite would head down to 67 degrees by Aug. 1, but they admitted that water temperatures were not behaving according to their model, which seemed stymied by the extra-hot water from Brownlee.

It's not the first time this season managers had gotten into hot water. Earlier in July, they had to change operations at McNary Dam after hot water from the Snake slid along the south side of the Columbia past their confluence and boosted temperatures up to 74 degrees in the fish bypass system at the dam. They said migrating juvenile salmon were highly stressed from passing into rapidly warming water (66 to 72 degrees) and began dying in large numbers by July 16.

On July 21, some fish managers requested a change in operations that would boost spill, reduce powerhouse flows and bypass fish back to the river on non-transport days.

The Corps agreed to spill to the total dissolved gas cap at McNary until barging began on July 24, when spill went back to 50-percent levels.

According to the fish managers' written request, up to 17 percent--about 11,000--of the young fall chinook collected for barging on July 18 died.

But most of the mortalities were hatchery fish. According to a July 24 Corps' memo, mortality of ESA-listed wild fall chinook during the July 16-22 period was estimated to be only around 400 smolts. At a generous 1-percent smolt-to-adult return rate, that would translate into less than four returning adults--more than 6,000 wild Snake fall chinook made it back to the Columbia last year.

In another memo that outlined the situation to federal BiOp judge James Redden, the Corps said if the temperature gradient in McNary's bypass system exceeded 3 degrees C from one end to the other, or if fish mortality exceeded 6 percent for three consecutive days, they would spill to the gas cap again until conditions improve.

By July 27, mortalities had dropped to about 1 percent after the operational changes were helped by declining water temperatures (to 70 degrees F). The third week in July is generally the hottest of the year on the Snake. After that, shorter days tend to help cool the river.

Luckily, the juvenile fish migration had dropped off considerably from its peak in early June. Only a thousand or so smolts were passing each lower Snake dam every day. The weather began cooling by the end of the week, so dam managers lowered the Dworshak outflow to 12 kcfs by Aug. 5. Flows from the Snake cooled down too.

But soon, adult fall chinook will begin showing up. They can benefit from cooler water temperatures, as well. There is already a considerable number of adult steelhead migrating upriver--thousands are passing Bonneville Dam every day.

If the Columbia gets too hot in August, adult chinook and steelhead tend to seek out cool-water refuges like tributary river mouths, where they will linger until water temperatures moderate.

Related Pages:
Dam Operators Fly Idaho Rivers, Map Hot Spots by John Miller, The Seattle Times, 8/9/9
Biologists at Bonneville Track Heat around Salmon Habitat by Tom Banse, KUOW, 8/14/9

Bill Rudolph
Dam Managers Tweak Reservoir To Aid Fish
NW Fishletter, August 18, 2009

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