Brownlee's Balance: Water level in Baker County's biggest reservoir depends on snowpack, flood control, other factors

Mar. 27—Baker County's biggest reservoir has shrunk quite a bit this late winter and early spring.

But Brownlee Reservoir, on the county's eastern border, likely would have been smaller still had the onslaught of storms over the past several weeks been aimed a bit farther to the north.

The 53-mile-long reservoir on the Snake River, which stretches from Huntington to Richland, is a key part of the system designed to prevent the Columbia River from flooding in the Portland area, more than 300 miles downstream.

Although Idaho Power Company owns Brownlee Dam, the company also coordinates with the U.S. Army Corps of Engineers to comply with flood control targets the agency sets, said John Hildreth, a senior engineer for Idaho Power.

Generally speaking, the deeper the snowpack in the Columbia Basin, the more water Idaho Power has to release from Brownlee to create room for the spring runoff.

The reservoir stores water that otherwise would swell the Columbia River.

This year, though, has been somewhat unusual, Hildreth said.

Although Idaho Power dropped Brownlee by as much as 54 feet below full pool earlier in March — when full the reservoir's level is 2,077 feet above sea level — meeting federal flood control requirements wasn't the main reason for the drawdown, Hildreth said.

Those requirements are actually quite modest by historical standards.

The Corps of Engineers has asked that Idaho Power keep Brownlee at least 10 feet below full by March 31, Hildreth said.

With Brownlee at 46 feet below full on Sunday, March 26, and slated to refill slowly, at approximately half a foot to one foot per day, Idaho Power will meet the March 31 target with more than 30 feet to spare.

So why is Idaho Power keeping Brownlee considerably lower than the flood control target?

The answer, Hildreth said, is that flood control isn't the only factor company officials consider in managing the reservoir during the late winter and spring.

More to the point, there are reasons other than flood control to lower the reservoir's level in preparation for the glut of water that will flow into Brownlee when the mountain snowpack begins to melt.

If Idaho Power met the flood control mandate precisely, with the reservoir just 10 feet or so below full now, there would be scarce room to accommodate the much higher flows on the Snake River later this spring when the river is swollen with snowmelt.

In that situation, Hildreth said, Idaho Power would have to divert a lot of water through the spillways at Brownlee Dam during April and May, rather than through the turbines that produce electricity. The reason, he said, is that there's a limit to the volume of water that can be sent past the turbines. Once that limit is reached, all excess water pours through the spillways.

The problem with sending lots of water through the spillways is that the resulting turbulence significantly boosts the concentration of dissolved gases, such as oxygen and nitrogen, in the river downstream, Hildreth said.

High concentrations of dissolved gas can kill fish through what's known as "gas bubble disease," according to research done for the federal government.

Diverting water through the power-producing turbines, by contrast, creates much less dissolved gas, Hildreth said.

Avoiding excessive gas concentrations isn't the only factor that influenced Idaho Power officials' decisions to lower Brownlee well below the federal flood control requirements, Hildreth said.

Maintaining space in the reservoir to collect snowmelt also helps the company avoid releasing large volumes in the Snake River through Hells Canyon, he said.

The two much smaller reservoirs downstream from Brownlee — Oxbow and Hells Canyon — lack Brownlee's storage capacity, so when Idaho Power spills a lot of water through Brownlee, flows in Hells Canyon rise rapidly.

That can cause localized flooding and pose a risk to rafters, boaters and other river users, Hildreth said.

"When you're completely full (in Brownlee) you're at the mercy of what's coming into the reservoir," Hildreth said. "We want to have that space in Brownlee to shave off those peaks in the river flows."

Although Idaho Power would have lowered Brownlee considerably this year due solely to the snowpack in Western Idaho, Hildreth said the reservoir wouldn't have dropped quite as much, to 54 feet below full, except for an unrelated issue.

The company is required by federal rules to periodically test the spill gates at Brownlee Dam, work that requires the reservoir level to be relatively low and that was done earlier this month.

If not for that project, Hildreth said, Brownlee's lowest level this year probably would have been about 5 feet or so higher than it was.

Significant variance in snowpack

Although the nearly incessant storms during March have pushed the snowpack well above average in the Western Idaho mountains where streams drain into the Snake River, those storms, which mainly were aimed at California and extended into Oregon, had much smaller effects farther north, in the upper Columbia River basin in Washington, Hildreth said.

As of Monday, March 27, the snowpack in Western Idaho was about 26% above average. In the upper Columbia River basin, by contrast, the snowpack was about 3% below average.

The Columbia Basin snowpack has a major effect on the risk of flooding, Hildreth said. And since the snowpack in that area is slightly below average, and the forecast flow of the Columbia River in Central Washington about 31% below average this spring and summer, the Corps of Engineers flood control targets are commensurately modest.

Much more modest than in past years when the snowpack was bountiful not only in Idaho but in the upper Columbia Basin as well.

"It's been quite a few years since we've seen such a disparity between (the snowpack in) the Columbia Basin and the Snake Basin," Hildreth said.

In April 1997 and April 1999, when the snowpack was well above average across the Northwest, the Corps of Engineers required Idaho Power to nearly empty Brownlee to make room for snowmelt and to reduce downstream flood risks.

In both months the reservoir dropped to 101 feet below full — 57 feet lower than Brownlee's low point this year.

Tom Conning, a public affairs specialist for the Corps of Engineers Northwestern Division in Portland, said that if forecasts for the spring runoff were higher, flood control targets would have been more significant.

Snowpack is higher in Western Idaho compared to the Columbia River Basin, which does contribute to modest flood risk management targets," Connins said.

Hildreth said he expects the flood control targets might have been a bit more substantial had the storms started in February rather than March.

The March 31 target for Brownlee, just 10 feet below full, was set at the start of the month, he said.

"Since then we've been getting hit by these atmospheric rivers, with 170% of normal precipitation in March," Hildreth said. "It's a dynamic situation."

The Corps of Engineers preliminary estimate for Brownlee's flood control target for April 15 is just 6 feet below full. Hildreth anticipates that when the official target is set this week, the figure will be somewhat more than 6 feet below full.

That won't affect Idaho Power, though, since Brownlee, at 46 below full, will easily meet the April 15 target even as the company slowly refills the reservoir.

Hildreth said the company's plan is to reach full pool by June 20.

The refill rate will average a half-foot to 1 foot per day, but the actual daily amount will depend on factors such as the Snake River's volume, he said.

"It's hard to predict when and how fast the runoff will come out," Hildreth said.

The reservoir's level has a major effect on recreation, and in particular whether Brownlee's many boat ramps are usable.

At the current level, about 46 feet below full, many popular ramps are out of the water, including Spring Recreation Site near Huntington (accessible when the reservoir reaches 22 feet below full), Farewell Bend State Park (40 feet), and Baker County's parks on the Powder River arm near Richland — Hewitt (39 feet) and Holcomb (30 feet).

Boat ramps accessible now include Moonshine Mine, on the Oregon side south of Richland, and Woodhead Park on the Idaho side.

Daily updates on the reservoir's level are available online at www.idahopower.com or by calling 1-800-422-3143.