The 67-foot-high Mackay Dam was built solidly, an earthen base with a concrete core when completed in 1919, wide and thick because builders intended to go higher before they realized there wasn’t enough water.
So it has stood up to floods that regularly run over its top, dynamiting by angry farmers during the 1933 drought and the 7.3-magnitude Borah Peak Earthquake with an epicenter 11 miles away in 1983. And it leaks at a rate of one cubic feet per second.
But even with the threat of an 80-foot wall of water just six minutes away from the 600 residents of Mackay, about 95 miles west of Idaho Falls, people there don’t worry. Not even as they watch news reports of the evacuation of 200,000 people this week from below the Oroville Dam in California.
“People know the dam survived the earthquake,” said Mackay Mayor Wayne Olson. “It actually made the dam stronger.”
Idaho Dam Safety officials call the dam a “high hazard,” which means its failure would present a threat to human safety and lives. It’s one of about a dozen older dams around the state that inspectors recommend for major repairs, knowing owners like the Lost River Irrigation District that owns Mackay Dam can’t or won’t pay the millions of dollars needed to bring the dams up to modern safety standards.
A state law approved in 2016 removed from state regulation any dam lower than 10 feet tall holding less than 50 acre-feet of water. That took 120 small dams off the state’s inventory, leaving about 435 dams that must be inspected every two years by state inspectors.
Half of those remaining dams are large, low-hazard dams, said John Falk, the Idaho Department of Water Resources Dam Safety Manager.
The other half are significant hazard (causing property damage if they were to fail) or high hazard (property and life). Idaho has identified about a dozen high-hazard dams in need of repair, although the list was not immediately available Tuesday, state officials said.
Federal dams, such as Lucky Peak east of Boise owned by the U.S. Army Corps of Engineers, and Arrowrock, farther up the Boise River and owned by the U.S. Bureau of Reclamation, go through a tougher, federal inspection regimen, said Michael Coffey, public affairs officer for the Bureau of Reclamation’s Northwest Region office in Boise.
“We have a pretty rigorous dam safety review,” Coffey said. Coffey said it’s been cited as a model around the world.
The bureau’s dam-safety program was forced to become stronger after its Teton Dam failed on June 5, 1976, killing 11 people and 13,000 livestock, and causing an estimated $2 billion in damages in Idaho Falls, Rexburg and other communities.
On Feb. 10, Falk sent a letter to the owners of all of the high-hazard dams in Idaho, recommending they begin releasing water gradually to make sure they have enough space for all of the runoff in their watershed this spring. He also told them to make sure that their spillways are clear of obstructions and that they have reviewed and updated their emergency plans.
But not all dam owners have updated emergency plans, Falk said. The state does not require them. In fact, state regulators don’t have much statutory authority to force irrigation districts and other dam owners to take any action except during an emergency — such as the one at California’s Oroville.
The state does have power, however, to restrict how much water dam owners may store in their reservoirs. In 2004, the state forced the irrigation company that owned the Fish Creek Dam near Carey to restrict its pool by half. The irrigators explored the cost of rebuilding the dam to allow them to store all their water again. The estimate was from $11 million to $20 million, Falk said, and the pool has remained restricted.
“If you mortgaged all the land under irrigation, you still wouldn’t have enough collateral for the bank to build the dam,” Falk said.
Preparing for more runoff from the Boise River watershed’s high snowpack, the Bureau of Reclamation and the U.S. Army Corps of Engineers increased flows from Lucky Peak Dam Monday to 750 cfs through Boise from 250 cfs. By Friday, they expect Boise River flows of up to 2,000 cfs through the city.
With the threat of catastrophic flooding in north-central California successfully crimped for the immediate future, the sheriffs here lifted mandatory evacuation orders in the early afternoon Tuesday, and within minutes thousands of those who had to flee over the weekend were eagerly streaming back to their houses.
The mandatory evacuation “order” for the estimated 188,000 affected people in Butte, Sutter, and Yuba counties was pulled back to an evacuation “warning,” clearing residents to return to their homes and businesses around 1:30 p.m., Butte County Sheriff Kory Honea said.
He said state engineers and work crews had managed to fortify the dam’s damaged emergency spillway enough with boulders and concrete so that it should withstand the series of storms expected to arrive Wednesday night.
Bill Coyle, acting director of the state Department of Water Resources, said the main spillway of the dam — also damaged, but not as badly as its emergency counterpart — was holding up well as it released a mighty roar of water at 100,000 cubic feet per second.