BOISE — Following an early May warm spell, water officials are concerned that river levels throughout the state are already orders of magnitude above usual peak flows, with most of an extraordinarily deep snowpack still remaining at high elevations.
On May 11, federal water managers said flood-control releases from Lucky Peak Dam hadn’t kept pace with increasing runoff into the Boise River, and they’d have to step up releases to levels that could damage downstream property. The Boise River Reservoir system was more than 81 percent full, with 149 percent of its normal mountain snowpack for the date still remaining, according to the USDA Natural Resources Conservation Service.
Mary Mellema, senior hydrologist at the Bureau of Reclamation, said the agency will increase flows through Boise by 300 cubic feet per second per day on May 15 and May 16 to reach 9,500 cfs. Anything more than 7,000 cfs is considered flood stage. Mellema explained prolonged wet weather has prevented BOR from making the volume of releases prescribed by an Army Corps of Engineers formula, called a rule curve.
At 9,500 cfs, the agency expects much of the Boise Greenbelt to be under water, exacerbated erosion of banks, flooding of Eagle Island, Garden City, Star and Caldwell, water flowing into 45th and 47th streets in the warehouse district of Garden City, some residential property affected and flooding of Warm Springs Golf Course and Municipal Park in east Boise.
Ada County emergency management officials said they’re re-enforcing river banks and will have personnel on the lookout for debris that could accumulate at bridges and quickly raise water levels.
“There’s been a lot of construction near the river since the last time we’ve had these kinds of flows,” said Brian Sauer, bureau water operations manager for the Middle Snake Field Office. “It’s a path we’ve never really been down.”
Complicating matters, the extended forecast for Southern Idaho calls for heavy rainfall, though the arrival of lower temperatures should slow the snow melt, according to the National Weather Service.
Though the Boise system is “several hundred thousand acre-feet behind the rule curve,” Mat Weaver, Idaho Department of Water Resources deputy director, said levels in the Upper Snake River system are right on target.
The Upper Snake Reservoir system was 65 percent full as of May 11. Palisades Reservoir was drained to just 12 percent of capacity in anticipation of peak runoff, with 162 percent of the system’s average snowpack for the date still remaining.
Lynn Tominaga, executive director of Idaho Ground Water Appropriators Inc., said his irrigators worry BOR drained Palisades too far. IGWA directed its attorneys to write the agency a letter encouraging judiciousness in future releases.
Lately, Steve Howser, general manager of Aberdeen-Springfield Canal Co., has heard similar concerns daily about over-drafting from his shareholders. But he recalls the 1997 season, when the Upper Snake had a similar snowpack and Palisades was left half full. That season, flood waters severely damaged his diversions. Howser is confident the system will still end this season filled to near capacity based on the extremely high river volumes and the remaining snowpack.
For example, the Snake River at the Shelley gage was at 22,221, cfs on May 11, compared with more typical flows for the date of just over 5,000 cfs. The Snake River at Flagg Ranch above Jackson Hole, Wyo., was flowing at 5,000 cfs. — five times the normal volume for mid-May, he said.