Jackson Lake poised for steep drawdown


JACKSON — When longtime Signal Mountain Marina manager Bill Wood returned to his administrative cabin overlooking Jackson Lake after taking last weekend off, he could immediately tell the water had changed noticeably in just 48 hours.

The level of the Tetons’ largest water body had dropped, seemingly abruptly.

“I felt like it went down, like, 10 feet,” Wood told the Jackson Hole Daily lakeside on Wednesday. “I know it didn’t, but it’s at 82 percent right now and when I left it was at 86 percent, so it went down 4 percent in two days.”

The sparsest water year for the Snake River watershed in a good while is primed to cause a last-resort and nearly absolute drawdown at Jackson Lake that the Bureau of Reclamation hasn’t had to execute in more than three decades.

That’s not to say that Jackson Lake will actually go dry.

There’s a glacially carved, approximately 400-foot-deep lake that predates modern civilization at the foot of the northern Teton Range. But more than a century ago it was impounded, and today the Jackson Lake Dam adds about 37 feet of depth — or 847,000 acre-feet of stored water — to the approximately 2 million acre-feet that would naturally slow at a wide, deep spot along the Snake River.

This summer 90 percent to 95 percent of that added water is expected to go downstream to Idaho, where it’s needed to fill Palisades Reservoir, which itself is being emptied out to keep American Falls Reservoir west of Pocatello from going dry. All that water is headed to irrigate potato fields in Idaho’s Snake River plain and grow crops that are used to nourish cattle parked on feedlots.

Driving the high demand are unseasonably warm temperatures and widespread drought, which coincided with an 80-percent-of-normal snowpack that melted off in a hurry. On Friday just 230 or so cubic feet of water was flowing into Jackson Lake every second, 29-percent less than the previous record low flow of July 9. But 5,250 cubic feet of water are leaving the lake through the dam every second, which is 22 times the volume coming in.

And so the lake drops.

“I’m drafting anywhere between 0.3 to 0.4 feet per day,” said Jeremy Dalling, who leads river and reservoir operations for the bureau’s Upper Snake Field Office.

Jackson Lake, in other words, is dropping between 3.5 and 5 inches every day. It’s losing 8,500 acre-feet daily, which pencils out to about 1 percent of all the stored water available. Although rain could change the equation, the pace of the drawdown could also increase slightly as the volume of Snake River water coming into the lake continues to slide.

“Physically, the reservoir in depth will reduce about 35 feet,” Dalling said. “We’re looking at pretty low reservoir levels from a historical standpoint. You have to go back to 1987-88 to see these types of reservoir levels that we’re talking about.”

That will change the look of Jackson Lake, especially northern reaches that are almost all influenced by the impoundment.

Generally, it’s also bad news for boaters. The surface of a totally full Jackson Lake is 6,759 feet above sea level. The end of the boat ramp near the Colter Bay Marina is at 6,753 feet, and at the current rate of decline just three weeks remain before the 60 boats anchored there for the summer will need to be trailered away. Notices have already been emailed out.

“It’s probably going to be by the end of the month,” Colter Bay Marina Supervisor Delaney Hunt said.

Hunt was hopeful that the public boat ramp would be operational for a bit longer.

Because their boat ramps are at lower elevations over deeper water, Signal Mountain (6,737 feet) and Leek’s Marina (6,735 feet) are less susceptible to an abbreviated season.

“Signal Mountain, we’ll probably see impacts late August, early September,” Dalling said. “If we were to encroach at Leek’s Marina that would be mid-September.

“Palisades is low, American Falls is low, everything is going to be low,” he added. “There are going to be impacts.”

At Signal Mountain, the lowest lake level since the 1980s is likely to snip about a full month off the boating season. Rentals were scheduled to cease for the year on Sept. 19, but private boaters who stored their vessel on a slip or buoy would have had until Oct. 3.

In the meantime, another factor — a workforce shortage — is already impacting Signal Mountain operations, according to the marina manager.

Although in a regular season seven employees work the marina, this summer it managed to bring on just four. As a result, rentals have been narrowed down to first-come, first-served kayaks and a couple of pontoon boats that are booked solid through the summer.

“In a regular season, we’d have six fishing boats, four runabouts, three pontoons and a deck cruiser,” Wood said. “We’re trying to figure this out.”