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The end of our flood? Runoff to spike this weekend, but Boise River could drop next week

Lucky Peak rooster tail water release

The "rooster tail" water release from Lucky Peak Dam on April 23rd, 2017 attracted thousands who came to see water shoot 150 feet into the air out of the dam's outlet side gates.
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The "rooster tail" water release from Lucky Peak Dam on April 23rd, 2017 attracted thousands who came to see water shoot 150 feet into the air out of the dam's outlet side gates.

Mary Mellema wouldn’t speculate about what day next week the U.S. Bureau of Reclamation might reduce the amount of water flowing out of the Lucky Peak dam and into the Boise River.

There are just too many variables to make that kind of prediction, said Mellema, a hydrologist for the Bureau. The most important variable is weather, she said. Right now, the forecast doesn’t anticipate major precipitation next week, but no one really knows because weather’s unpredictable.

Barring significant precipitation, though, Mellema said the Bureau could start reducing its discharges from Lucky Peak by late next week.

[Related: Flooding 101: Why the Boise River overflows its banks, and will again and again]

Before we get there, the amount of water flowing into Lucky Peak and the other two reservoirs on the Boise River system is likely to spike, she said.

As of Tuesday, the Bureau calculated a total of about 14,500 cubic feet per second of inflows to the Lucky Peak, Anderson Ranch and Arrowrock reservoirs. By Friday, boosted by recent warm weather, that should climb to about 15,000 cfs and peak on Saturday in the mid-15,000s, Mellema said.

As a precaution, Boise City has installed a "Muscle Wall" to keep potential flood waters from the Boise River from entering Zoo Boise. About 2,000 feet of interlocking panels stretch along the greenbelt path from the band shell to the tennis court

The Bureau doesn’t expect to increase flows out of Lucky Peak this week despite additional inflows, she said.

By Monday morning, Mellema expects inflows to eat up about half the roughly 49,000 acre feet of space left in Lucky Peak, Arrowrock and Anderson Ranch, leaving around 24,000 acre feet of space.

[Related: Will I ever be able to float the Boise River this summer?]

An acre foot is the amount of water it takes to cover an acre with one foot of water. It is equal to almost 326,000 gallons. Lucky Peak, Arrowrock and Anderson Ranch can store a total of almost 950,000 acre feet.

As spring progresses into summer, the chances that the Bureau can safely reduce Lucky Peak’s discharge increases. In large part, that’s because there’s less snow to melt into the runoff that ultimately flows into the reservoir.

“The snow line keeps getting farther and farther up into the mountains. Not only does it take hotter temperatures to get it to run, but it’s got a long way to run to get into the reservoirs,” Mellema said. “So, even with warmer temperatures, we’re not going to see any kind of inflows get as high as we’re forecasting for this weekend. But, you know, there’s always a chance of precipitation. So we never say never. If there’s significant precipitation in the Boise basin, the inflows could spike up again. But right now we’re not anticipating that.”

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