Reservoirs in the U.S. Bureau of Reclamation’s Upper Snake River system likely will contain good water supply ahead of the 2020 irrigation season.
Carryover storage — water that goes unused one year and thus is available for irrigation and other uses the next — is projected to be at or above the 30-year median in Reclamation tributary reservoirs above Milner Dam, said civil engineer Brian Stevens, water operations manager for the bureau’s Upper Snake Field Office in Heyburn, Idaho.
“We should have really good storage going into next year,” he said. “If we have a dry winter, having good storage in late winter and early spring means we will have good storage into next year.”
Combined, the reservoirs are expected to be 51% full, or 126% of the 1981-2010 median, as of Nov. 1. That's about the same as a year ago, Stevens said. Reclamation projects the reservoirs at 85% full as of March 1. Privately operated Henry’s Lake is not included in forecasts.
Reservoir levels last year peaked later due to heavy rain in June. This year, a cool, wet spring reduced early irrigation demand.
“Both 2018 and ’19 had long, warm and dry summers with similar irrigation demand,” Stevens said.
He told the Idaho Water Resource Board’s Upper Snake River Advisory Committee Sept. 4 that Reclamation expects Wyoming’s Jackson Lake to be 71% full Nov. 1 and 76% full March 1, the winter limit to allow for snowmelt and any subsequent flood-control releases.
In southeastern Idaho between Nov. 1 and March 1, Palisades Reservoir should go from 58% to 84% full while American Falls Reservoir is expected to go from 30% to 92% full.
American Falls — downstream from Jackson and Palisades and above the Burley area’s Milner Lake — is a major delivery point for many irrigators in southeastern and part of south-central Idaho.
“It can fluctuate more than Palisades or Jackson,” Stevens said. “It’s typical for American Falls to get fairly low at the end of every year" due to water deliveries.
American Falls reservoir levels often are low at the end of the water year so Palisades and Jackson can keep as much water as possible, he said.
“If we send water out of Palisades too early, it could spill out of American Falls before irrigation season really comes on,” Stevens said. Keeping water longer in Palisades and Jackson reservoirs also aids power generation.
With around a month left in irrigation season, Reclamation pegged Palisades and Jackson at 70% and 80% full, respectively. American Falls was 24% full.
“American Falls looks low now, but given how full Palisades and Jackson are, we are in a good spot water-wise in the basin,” Stevens said.