News

Nov 22, 2023

Stuck headgates put Mackay Dam in state of ‘uncontrolled release’

Mackay Dam’s spillway—shown here on the left—is in great need of repair, according to the Big Lost River Irrigation District. However, the more pressing concern is two underwater headgates that can no

Mackay Dam’s spillway—shown here on the left—is in great need of repair, according to the Big Lost River Irrigation District. However, the more pressing concern is two underwater headgates that can no longer close. Pictured to the right is the dam’s plunge pool leading into the Big Lost River.

The failure of two 105-year-old underwater gates on the Mackay Dam last month has prevented the Big Lost River Irrigation District from being able to release water from Mackay Reservoir in a controlled manner, causing flows to rush into the Big Lost River at nearly twice the normal rate for this time of year.

In a Wednesday interview, Irrigation District Manager Kevin Whitcomb said that two of the five release gates got stuck in early to mid-June. The district now has no choice but to let the reservoir drain down through August and make necessary repairs once the dam is empty.

Currently, water is emptying into the Big Lost River at about 900 cubic feet per second, Whitcomb said. Normally in late July, the outflow would be closer to 500 cubic feet per second.

“Nine-hundred CFS is definitely more than we want to lose,” he said.

Mackay Dam—which is owned and managed by the Big Lost River Irrigation District—holds back some 10 to 14 billion gallons of impounded water, mostly used for irrigation by farmers and ranchers in the Mackay area. The earthen dam was built in 1918 by Utah Construction Company, which later built the Hoover Dam in 1931.

Currently, any excess flow from the reservoir is discharged down a flat concrete channel called a “spillway,” which is intended to safely pass floodwater over the crest of the dam and into the Big Lost River on the other side.

About 50 yards away from the spillway’s intake ramp along the shoreline is a buoy-shaped concrete “tower” that sits atop a 60-foot-deep underwater drain with five release gates. From the inlet tower, an operator can mechanically control how much water is fed from the reservoir through the underwater gates below his or her feet.

“Of the five gates, three are at the 60-foot level and two higher up,” Whitcomb explained. “At the top of the tower is an electric motor that you attach to the gears of the gate you would like to raise or lower. We can’t get two of the gates to move up or down. It’s brass on brass.”

In September, a team of engineers will extract the broken head gates and use a “white poly” plastic to line the gates and tower opening, creating a strong seal, Whitcomb said.

“The (new gates) will wear much better than steel or brass, and won’t freeze—we’ve always had trouble during the winter with those doors freezing shut,” he said.

Whitcomb said he was unsure how much the repairs will cost in total but said the district will be using its $2 million “aging infrastructure” grant award that it received from the Idaho Water Resource Board in September 2022.

Whitcomb said the district considered hiring a hard-hat diver to repair the two gates but decided it would be too dangerous due to the suction pressure near the gates’ screening. He expects the reservoir—currently 70% full—to hit the 30% mark on Aug. 15 and the 10% mark on August 28. At the 6% mark, Whitcomb said the dam will be considered empty, and repair work can start. Fish will still survive at that level, he said.

Whitcomb added that a good portion of the excess water is disappearing into the Lost River Valley aquifer about 5 miles below the dam. In a few years, he said it should make its way down “by gravity” all the way south to the Perrine Bridge in Twin Falls, “kind of like a spring coming right out of the side of the embankment of the (Snake) River,” and benefit growers in the Magic Valley region.

Both the Idaho Department of Water Resources and the Idaho-based Environmental Defense Institute have found that the Mackay Dam could suffer catastrophic failure due to long-term erosion and its location along a fault line. In 2018, the Environmental Defense Institute declared the dam a “clear and present danger” to Mackay, Arco, and the Idaho National Laboratory, warning that a total failure could send an 80-foot wall of impounded reservoir water south into Mackay within six minutes—potentially claiming lives.

On Wednesday, Big Lost River Valley resident Joe Chlebowski, formerly of Ketchum, said he thought that was a possibility.

“In six seconds, we’d have at least a 20-, 30-foot wall of water moving at 50,000 cubic feet per second,” he said. “I live about 4 miles below the dam. It would indeed be catastrophic.”

Whitcomb, however, said any concerns about the dam collapsing are unwarranted.

“My biggest concern now is for the farmers and ranchers down below the dam,” he said. “This may prevent them from having a second, or third, growing season.”

One rancher who receives his irrigation water from a canal off the Big Lost River and spoke to the Express on Thursday on the condition of anonymity said that much more water is coming down the dam than needed. From a farming standpoint, it’s a massive waste of water, he said, but there’s not much that can be done.

“This is going to affect the whole valley, dry the whole valley up,” he said. “A lot of it is incompetence of the Irrigation District. I know they were told not to touch the gates last spring and did it anyway. But it is what it is.

“They’ve got gates that won’t close, I understand, but the stories about ‘dam failure’—that’s pretty much all hogwash.”

After the gate work this fall, Whitcomb said several more repairs to the spillway are needed. Right now, some water is leaking out of the steel conduit pipe before it reaches the plunge pool and seeping into the embankment, he said. The spillway concrete is also weathered and cracking, and cliffs around the spillway will need to be wrapped in rock netting to prevent boulders from dropping onto the concrete.

More immediately, the headgate failure has had some consequences for local businesses. Scott Schnebly, owner of Lost River Outfitters in Ketchum, said his employees will be unable to guide on its namesake river for the rest of the season because fishermen can’t safely wade in the river at the current level of release.

In all, Schnebly said he’s expecting to lose “about 100” guide days.

“We have lost about 50 so far,” Schnebly said. “We have not been advised of what happened to the gates, the plans of repair, status of repairs, or potential repercussions of the drawdown. Everyone seems to be in the dark.”

Schnebly said he expects some clients to cancel. Others will fish a substitute waterway and probably won’t book, he said. As of now, it’s difficult to estimate losses until it’s time to do end-of-year reports.

“Many clients have traveled from the East Coast, booking multiple-day trips, asking for at least one of their days to be on the lower Lost waters,” he said.

Schnebly noted that the bottom-release Mackay Dam helps create ideal fly-fishing conditions because it keeps the water temperature so consistent. He said the Big Lost is one of the best fisheries in the western U.S.—a spot with “more 20-inch rainbows per capita than any other waters in the state,” he said.

“Being a tailwater fishery, water temps are fairly constant. That helps keep the fish feeding on a long-term regular schedule,” Schnebly said. “Such consistency is also good for healthy aquatic bug populations. Those are some of the best fed trout in the state.” 

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“This is going to affect the whole valley, dry the whole valley up."

Big Lost area rancher

Obvious someone is not doing their job.

Maybe Idaho should fix dams and schools with our budget surplus instead of giving property tax relief to the wealthy.

to the wealthy? how about just the average idahoan, i dont want more of my taxes going to fed schools.

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