The U.S. Bureau of Reclamation began flood-control releases at Palisades Reservoir April 7-10, which enabled the Idaho Water Resource Board on April 11 to resume Eastern Snake Plain Aquifer recharge operations, at least temporarily.

The moves indicate water supply is good as the irrigation season gains momentum.

Palisades is the second farthest upstream of the nine Upper Snake River reservoirs. Jackson Lake, in Wyoming, is the farthest upstream. Milner Reservoir, the farthest downstream, has zero outflow as irrigation season starts because all water from that point upstream is allocated.

Above-normal snowpack prompted Palisades flood-control releases and made excess water — over and above what is needed to fill rights to water in the Upper Snake reservoir system — available to release from Milner, officials said. How long Milner outflow is available to be returned to the aquifer will depend on weather and early season irrigation demand.

“Flows are expected to vary over the next few months for Palisades as it goes through meeting its flood risk management requirements,” said Brian Stevens, water operations supervisor with Reclamation’s Upper Snake Field Office in Heyburn, Idaho. Operations will be dynamic and dependent on conditions.

Snowpack in the region is above the long-term median, and Reclamation forecasts 102% of normal runoff from April through July, he said.

“There is excess water in the system not able to be stored, and that’s driving some spill at Milner,” which “turns on additional recharge opportunities in the system,” Stevens said.

Milner outflow for aquifer recharge will last until there is no more water in the Upper Snake reservoir system in excess of what is allocated, he said. That timing “will be dependent on irrigation demand and runoff in the system and reservoir management.”

During winter months when the Water Resource Board’s recharge right is active, “there is a very limited amount of water available for us to recharge in the lower valley,” said Wesley Hipke, who manages the board’s recharge program. “There is a need to recharge more, and it is completely dependent on flood releases.”

As flood-control releases go, “it doesn’t appear it’s going to be a large amount, and there are questions about how long it will last given that irrigation season has started,” he said.

Nevertheless, the Milner outflow represents excess water that otherwise would leave the state, “and we want to use that to help reach our goals,” Hipke said.

“I’m always impressed with how willing and how much the parties work together to make this work,” he said.

The Eastern Snake Plain Aquifer, which lies underneath much of the state’s eastern and south-central regions, has been declining for decades for reasons including population and business growth and efficient irrigation systems that return less water. IWRB has operated a voluntary annual recharge program since 2014.

Flood-control operations at Palisades typically end in middle to late June, Stevens said. How this year’s operation unfolds will depend on how the snow melts and on future weather conditions.

At some point, the flood risk management operation ends because it calls for less flow than irrigators demand, he said.

Irrigation season, which began in late March, “has been steadily increasing, and will continue to increase until approximately late June, when it normally reaches its yearly maximum for diversions,” Stevens said.