Letters from the West

Cool temperatures could give Boise River flood fighters the break they need

Temporary levee construction begins on Boise River

A U.S. Army Corps of Engineers team is seen here constructing a temporary levee along the Boise River in the area near Eagle Island. Crews are building a 4,300-foot levy along the Boise River, west of Glenwood Street, to divert flood waters away
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A U.S. Army Corps of Engineers team is seen here constructing a temporary levee along the Boise River in the area near Eagle Island. Crews are building a 4,300-foot levy along the Boise River, west of Glenwood Street, to divert flood waters away

Freezing temperatures in the mountains is slowing the inflows into the Boise River reservoir system and the U.S. Weather Service is predicting continuing cool temperatures over the next two weeks.

That could give the U.S. Army Corps of Engineers and the Bureau of Reclamation a chance to increase the space in those reservoirs and more breathing room to help get the Boise River through the runoff without major flooding. Inflows into the reservoirs have dropped from 12,000 cubic feet per second Sunday to 9,755 cfs Tuesday morning.

Inflows could drop to as low as 8,500 cfs over the next couple of days, said Jay Breidenbach, U.S. Weather Service warning coordination meteorologist in Boise.

“The cooler, drier weather is helping,” Breidenbach said.

Rocky Barker explains the basic reality of this year's flood season on the Boise River (Rocky Barker)

Less than a half inch of precipitation fells as snow on the Boise Basin over the weekend, which was great for Bogus Basin skiers, he said. But it kept the snowpack in the watershed at about 2 million acre-feet of water.

The outflow at Lucky Peak is 9,066 cfs, still less than the inflow into the reservoir system. Irrigation will begin to pick up, supplanting the 1,184 cfs going into the New York Canal to fill Lake Lowell, which is at 95 percent. Some water has been passed through the lake into the Snake River.

Water managers have said they plan to increase flows from 8,100 cfs at Glenwood Bridge to 8,600 cfs by Wednesday. Flows were 8,016 cfs Tuesday morning. Workers are completing a temporary levee to protect the Sunroc gravel pit Tuesday morning from “capturing” river flows and causing area flooding near the head of Eagle Island.

Steve Hall, U.S. Army Corps of Engineers water management program manager, said April 6 that federal hydrologists and engineers had analyzed 22 runoff seasons on the Boise River since Lucky Peak was built in 1955. Given current conditions, the Valley would have gotten through the runoff to July 1 without serious flooding in all but two of those years. Those two years, he described as “trouble.”

From the spillway below Lucky Peak Dam to washed-out bridges near Idaho City, the impact of high water is on display.

Breidenbach said the forecast for the next two weeks was for below-normal temperatures and rain with snow in the mountains. But if the nights remain under freezing in the higher elevation, dam managers may be able to increase storage space with continuing releases from the reservoirs.

“There are scenarios that get us out of this without too much trouble,” Breidenbach said. “I’m feeling pretty good about the forecast at this time.

“But you don’t want to pull the wild card,” he said. “Hot temperatures and rain would be the wild card.”

Rocky Barker: 208-377-6484, @RockyBarker

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