A cool and damp spring has prolonged the melting of an above-average mountain snowpack throughout much of Idaho.
Consequently, water managers are optimistic they’ll draw natural flow rights from rivers much longer than normal, potentially contributing to strong storage carryover into 2015.
Ron Abramovich, water supply specialist with the Natural Resources Conservation Service, said recent storms pushed the Lochsa and Selway river watersheds of the Clearwater Basin to 200 percent of normal as of May 12. In the Upper Snake system, tributaries in Wyoming ranged from 169 to 178 percent of normal snowpack.
The Boise River system had 119 percent of normal snowpack. Abramovich visited Bogus Basin in Boise following last weekend’s storms and found a turn toward colder weather had frozen the snowpack solid, and fresh snow had fallen.
“That’s ideal for pushing natural flows out later,” Abramovich said. “It’s shaping up to provide better carryover into next year the way the snow is coming off.”
With so much snowpack still in the mountains, Abramovich said the National Weather Service and U.S. Bureau of Reclamation will be monitoring melt rates and storage levels closely due to the potential for high river flows. Fortunately, he said there’s plenty of space to capture the melt in reservoirs. Jackson Lake and Palisades Reservoir are both 35 percent full.
“The good news is we’ve got a good snowpack in the mountains, so reservoirs should fill,” Abramovich said.
Steve Howser, general manager of Aberdeen-Springfield Canal Co. in southeast Idaho, said throughout the past 35 years, July 17 has been the average date that his company has switched from natural flows to storage water.
When he ran this season’s data through a spreadsheet, he calculated he should make it into mid-August on natural flows. Howser acknowledges a major rainstorm falling on snow could change circumstances.
“Right now, I’m looking at getting through peak use on natural flows,” Howser said. “If that’s the case, I suspect I’ll use 30 percent or less of my normal storage use. If the reservoirs are full, that would leave me two-thirds of my normal space in carryover.”
Abramovich emphasized some parts of Idaho were missed by storm systems and have below-average snowpacks. The cold weather has somewhat delayed snowmelt in the Owyhee Basin of southwest Idaho, but Abramovich said the water outlook is still poor, with a snowpack of 61 percent of average. He said streamflows are projected at 48 percent of normal through September, and reservoir storage levels have already peaked for the season in Owyhee Dam.
The Oakley and Salmon Falls areas of Southern Idaho received more than half of their normal May precipitation by May 9, but the basins still face significant moisture deficits, Abramovich said. In Idaho’s central mountains, he said the Big Wood and Little Wood systems have 15 percent of normal snowpack, and the Big Lost basin is at 64 percent of normal.