Boise River flows are slated to increase starting March 13 to reduce flood risk.

The U.S. Army Corps of Engineers, working with the U.S. Bureau of Reclamation, plans to increase Lucky Peak Dam outflow to 750 to 1,000 cubic feet per second, said Ryan Hedrick, hydrologist with Reclamation’s Middle Snake Field office in Boise. Outflow has been about 240 cfs — a winter minimum set to accommodate fish and wildlife — since irrigation season ended Oct. 9.

Of the three Boise River dams, Corps-owned Lucky Peak is the farthest downstream and closest to the city of Boise. Reclamation owns Arrowrock and Anderson Ranch dams upstream.

The system as of March 6 contained 560,000 acre-feet of water, about 110% of average, Hedrick said. Inflow from snowpack is expected to be 120% of average, according to a March 1 forecast coordinated by the two agencies.

“We are going to begin to create space in the reservoirs for the inflow,” he said.

Depending on weather, the elevated releases could continue and could increase until risk is reduced, Hedrick said.

How long the operation will last is yet to be determined, he said. Flood-control operations often wind down from late May to mid-June as runoff decreases.

“The irrigation outlook is good,” Hedrick said. “The reservoir systems should all fill.”

Star-based Water District 63 delivers Boise system irrigation water to right holders and tracks how much they use.

Flood-control releases “help the water district in that all irrigators who have (reservoir) storage start with a full allotment,” WD 63 watermaster Daniel Hoke said. That helps farm irrigators in planning and growing their crops without having to worry about falling short of water, “barring something unforeseen.”

Winter outflow from Lucky Peak is 245 cfs, a function of a water right that Reclamation shares with Idaho Fish and Game. A sizable supplier of drinking water in the Boise area has 5 cfs. River flow increases to about 300 cfs at Glenwood Bridge in Garden City due to natural gains over the 20-mile stretch, Hedrick said.

Increased releases to start March 13 target 1,000 cfs at the Glenwood gauge over the March 15-16 weekend, according to a Corps update circulated by email. Operations from March 17 onward will be decided based on future basin conditions and forecasts.

“Since future weather can still highly influence the final peak snowpack and its runoff, there is still a large variability in the actual runoff volume this spring,” according to the update. After analysis, the Corps “concluded that releases must begin in mid-March in order to safely respond to all realistic scenarios.”