The Upper Snake River reservoir system is about 48% full, which is at the 1991-2020 median but 4% lower than a year ago, according to the U.S. Bureau of Reclamation.

“Even though we had almost 100% fill of the reservoir system in 2024, the hot, dry summer resulted in lower carryover for this year,” said Jeremy Dalling, water operations civil engineer with Reclamation’s Upper Snake field office in Heyburn, Idaho. Irrigation demand was strong, leaving less water stored at season’s end.

The biggest year-to-year change involved the two headwater reservoirs, he said.

The farthest upstream, Jackson Lake in Wyoming, was 76% full and 131% of the median as of Nov. 17, about 8% higher than a year ago, Dalling said. A drawdown for inspection occurred in 2023, reducing volume.

Downstream in eastern Idaho, the next reservoir, Palisades, is 42% full and 82% of the median. That is 21% below a year earlier due to Jackson’s higher volume and strong demand for irrigation water this year, he said.

“Reclamation endeavors to keep water in upper reservoirs to retain operating flexibility,” Dalling said. As for Jackson’s higher volume, “we like that number. Jackson and Palisades basically function together.”

Lower volume at Palisades is “something to track,” he said. “With an average snowpack, runoff could offset the distribution between the upper and lower system.”

Estimates by USDA’s Natural Resources Conservation Service suggest 95% of normal runoff is needed for a full water supply in the Upper Snake reservoir system. Dalling and NRCS staff spoke at an Idaho Department of Water Resources supply outlook meeting Nov. 19.

Volumes in other Upper Snake reservoirs are “at similar levels to a year ago but experiencing slightly lower refill rates due to lower natural flow,” he said. These include Ririe, Island Park, Grassy Lake, American Falls, and Minidoka Dam that impounds Lake Walcott. The privately owned Henry’s Lake also has a volume similar to that of a year ago “and is in good position to refill.”

At Milner, the farthest-downstream dam in the Upper Snake River system, available flow is being returned to the Eastern Snake Plain Aquifer under the state’s natural-flow water right.

Reclamation is slated to issue a runoff forecast in January.