American Falls Dam

The American Falls Reservoir is full, according to the Bureau of Reclamation. Ample water will be available in most of Idaho for irrigation and hydropower.

Half of the Upper Snake River Basin’s 10 reservoirs in Idaho and part of Wyoming were full on June 4, the U.S. Bureau of Reclamation reports.

Altogether, the system was 89% full, with several reservoirs likely to receive more inflow as the weather warms. Irrigation supplies should be ample.

Henry’s Lake, Grassy Lake, Island Park, Ririe and American Falls reservoirs were full. Lake Wolcott and Little Wood reservoirs were at 98%, Jackson Lake 86%, Milner 76% and Palisades 73%.

Brian Stevens, supervisory civil engineer for Reclamation’s Upper Snake Field Office in Heyburn, said at a June 5 meeting of area water managers that the June 1 runoff forecast for the Snake River above Heise, site of the main regulatory gauge for Palisades Reservoir in eastern Idaho, was 108% of average. Two similar years for runoff volume were 1991 and 2014.

“System inflows in those similar years tapered off after about the third week of June,” Stevens said in an interview. “Reclamation is watching inflows to mange flood-control operations.”

Runoff has been slower recently in some spots due to lower-than average temperatures, but it is starting to increase, he said. For example, Palisades — which, like Jackson Lake in Wyoming continues to run flood-control releases — is filling quickly.

“We are targeting to fill as much as flood-control operations will allow at Palisades in the next three weeks,” Stevens said.

He said Reclamation does not expect to spill excess water from Milner, at the downstream end of the Upper Snake system, but by early July likely will start releasing water for flow augmentation to benefit fisheries far downstream.

Reclamation anticipates irrigation demand to increase significantly over the next couple of weeks “back up to normal irrigation diversion levels,” Stevens said. It has been down recently due to cool, wet weather since about mid-May.

He said the bureau expects the reservoir system to keep gaining water for storage until late June, when maximum storage content is reached. That should mark the start of major seasonal “drafting” of reservoir water for delivery to irrigation districts.

Forecasts through early fall anticipate warmer and wetter conditions, said Kurt Buffalo, Pocatello-based meteorologist at the National Weather Service.

Stevens said precipitation during peak irrigation season will not change demand by nearly as much as it fell from May to June.

“We anticipate irrigation demand to be at least average, or just above, this year,” he said.

Stevens and Buffalo spoke at an Upper Snake River Advisory Committee meeting at the Idaho Department of Water Resources.

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