Jackson Lake Reservoir filled on Tuesday and releases from Jackson Dam will slowly be reduced, a Bureau of Reclamation official said Thursday.

The reservoir in Grand Teton National Park is now holding its maximum 847,000 acre feet, water impounded for and owned by irrigators in Idaho. Last year it filled May 22.

Bureau workers will now verify the inflow and its declining trend, then adjust dam releases to drain 200,000 acre feet from the reservoir by the end of September, said Mike Beus, a Bureau official in Idaho.

The Snake River on Thursday was adding to the reservoir’s volume at 2,920 cubic feet per second, according to a reclamation Web site, while the dam was releasing water at a rate of 5,681 cfs.

Boaters on Jackson Lake can expect the shoreline to remain relatively stable for now, Beus said.

“It’s going to be within a foot for a few weeks,” he said of the water level. “It will be dropping a little faster as we get into September.”

The reservoir must have at least 200,000 acre feet of storage available before winter to comply with the Bureau’s flood control program. That translates to a drop of approximately eight feet from the current, maximum reservoir surface elevation, Beus said.
“We would go below that if irrigators need it,” he said. However, “There’s not much risk we’ll have to take more than 200,000 out this year.”

The flow pattern is wildly different from last year’s, when the Bureau had to dump a lot of water out of the reservoir to meet irrigation demands in late summer. That put off fishing on the Snake during the prime months of August and September.
“Last year was exceptional in total volume of storage used,” Beus said. “We went from a nearly full system to a nearly empty system. It was a long season.”

In contrast, the bureau is still storing water this year on July 4 – a late date. “Demand this season will be two months shorter,” he said.

Beus said valley irrigators’ rule of thumb – when the snow goes off the belly of the Sleeping Indian the runoff has peaked – appears to be holding. A small snow patch remained Thursday.

“The Gros Ventre is performing consistent with the rule of thumb,” Beus said. “Its timing is pretty consistent. The snow will be depleted very soon; inflows are going to drop dramatically.”

Palisades Reservoir is “fuller than we could have hoped,” Beus said, but American Falls Reservoir, farther downstream in Idaho, has space in it.