Palisades Reservoir on the Upper Snake River in eastern Idaho.

Upper Snake River reservoirs are 67% full, up from 34% a year ago — a bright spot in this year’s water-supply outlook as snowpack remains well below normal.

The system’s current volume is about 114% of the 1991-2020 median, according to officials with the U.S. Bureau of Reclamation’s Upper Snake Field Office in Heyburn, Idaho.

Runoff from January through July 2023 was 99% of normal at the Heise gauge, northeast of Idaho Falls, up from 73% in the year-earlier period, said Brian Stevens, Reclamation water operations supervisor for the region.

“It was a really good runoff year last year,” he said. “It resulted in the large increase in storage last year to this year.”

For example, the strong runoff at Heise resulted in 660,000 acre-feet of additional volume stored in Palisades Reservoir, and an additional 419,000 acre-feet in Jackson Lake. That lake, in Wyoming, is the farthest upstream of the eight Upper Snake storage reservoirs.

Palisades, in Idaho, is next downstream.

Runoff in 2023 was 97% of average on Henry’s Fork of the Snake near Rexburg, Idaho. Runoff was an observed 141% of average in the Portneuf River, to the south near Pocatello.

American Falls is the Upper Snake system’s largest reservoir. Milner Dam is the farthest downstream, and the system’s last diversion point.

Given the good 2023 runoff in the upstream areas as well as points downstream, “American Falls Reservoir may not be able to contain all the water that flows into it this spring,” Stevens said. “So we may see water spill past Milner. But all of that depends on several factors yet to unfold.”

Above-average runoff and reservoir storage “raise the likelihood of unstorable water being spilled past Milner Dam this spring,” he said.

Current snowpack, however, is well below average — 61% in the Upper Snake Basin. It would shorten the duration of potential water spill, he said.

The snowpack “is a factor in potentially delayed flood-risk management this year compared to last year” at Palisades, Stevens said.

Downstream, potential water spill past Milner thus would be “relatively brief in comparison to large runoff years,” he said.

“There’s a lot left to go, and a lot left to happen this season,” Stevens said.

The water year in the Upper Snake started with a wet October, which was followed by a dry November that reduced water-year rain and snow by more than 30% in all three sub-basins, according to USDA’s Natural Resources Conservation Service. December precipitation was mixed.