The over 100-year-old Mackay Dam in Idaho’s Custer County needs repairs and poses a risk to the town of Mackay just downstream and the Idaho National Laboratory about 30 miles further, according to the Idaho Department of Water Resources and the Environmental Defense Institute, an Idaho nonprofit focused on nuclear energy issues.
Mackay Reservoir, owned and maintained by the Big Lost River Irrigation District, stores water primarily for irrigation and agriculture, with a maximum storage of 45,000 acre feet of water. However, the age and lack of repairs to the dam have increased the risk of its failure in the event of flooding or an earthquake, according to the Idaho Department of Water Resources, the Environmental Defense Institute and the Idaho National Laboratory, where nuclear waste is stored and nuclear energy research is conducted. Both natural disasters have a history in the region, and the dam sits near the Lost River fault and Borah Peak.
From spillways to gates, Mackay Dam needs repairs
The list of needed repairs for the structure is long, according to regular reports from the Department of Water Resources. The dam’s emergency spillway is too small and deteriorating. All five of the dam’s gates are in various stages of decline, and two are unable to be used. A leak at the foot of the dam could contribute to increasing erosion of the concrete of the dam itself, though the leak has not shown any signs of major erosion yet. The dam lacks a warning system in the event of a dam failure, human or electric.
Central Idaho does have a recent history of high precipitation, which has resulted in Mackay Dam overtopping three times in the last 15 years: in 2010, 2011, and again in 2017, according to a 2018 complaint to then-Gov. Butch Otter. The overtopping event in 2017 came after warnings for high amounts of snowmelt from the National Weather Service, the Challis Messenger reported.
In a 2017 letter from Idaho Department of Water Resources Director Gary Spackman to the Big Lost River Irrigation District, Spackman warned that the high snowmelt levels would fill the reservoir faster than it could be emptied, therefore increasing the erosion to the already damaged spillway as a result of exceeding maximum flow rates.
Spackman asked the irrigation district to increase the amount of water being let out of the dam. Due to a river breach in a levee near Leslie, the Big Lost River Irrigation District refused to increase the amount of water being let out of the dam because it would have caused flooding.
Regional earthquakes may cause Mackay Dam failure
Earthquakes also pose a large risk to the integrity of the dam because the seismic ratings of the construction are unknown. The Mackay Reservoir was constructed around 1921 by the Utah Construction Company, which helped in the construction of the Hoover Dam outside Las Vegas in 1931. The company built the Mackay Dam toward the beginning of its dam-building days and didn’t have much previous experience with dams, according to the Environmental Defense Institute.
Concerns over the structural integrity of the dam and how it would hold up in different scenarios are not new. The Environmental Defense Institute’s 2018 complaint said the dam was unsafe and a ‘clear and present danger’ to nearby towns.
“From my perspective, it’s an accident waiting to happen,” said Dave McCoy, who previously worked for the Environmental Defense Institute and helped write the complaint the organization submitted in 2018.
Chairman of the Big Lost River Irrigation District’s board Byron Pherson said despite the Environmental Defense Institute’s claims that the construction of the reservoir wasn’t well documented, the irrigation district has blueprints and other documents detailing the construction of the dam.
Under recent rulemaking being considered by the Department of Water Resources, which would decrease regulations on dams and mining operations with the risk of decreasing safety, according to the Environmental Defense Institute, seismic ratings for dams are not specified. Many of the regulations would be left up to the director of the Department of Water Resources, according to the draft of the new regulations.
In 2020, a 6.5 magnitude earthquake rocked Custer County and a state of emergency was declared after minor damage to the foundations of some buildings across Custer County was observed, though the dam didn’t suffer any damage according to an inspection letter from the Department of Water Resources. A resident told the Challis Messenger that the 2020 quake brought back memories of the 1983 Borah earthquake, which was a magnitude 6.9.
“(The Big Lost River Irrigation District’s) seismic analysis and understanding of the spillway, even when it was constructed in the ‘50s, is totally absent and inadequate,” said Tami Thatcher, who writes about nuclear issues for the Idaho National Laboratory and writes for the Environmental Defense Institute. “Then, after the 1983 Borah earthquake, the dam survived, but they were concerned about the rock cliffs above the spillway. The next year they hired someone to come in and blast rocks.”
Repairs on Mackay Dam won’t be cheap, and funding has been hard to get
All of the repairs needed to make the dam more stable will cost millions of dollars, and Idaho Department of Water Resources Dam Safety Manager John Falk said that price tag will only keep going up.
Pherson said it is difficult for the small communities in the area to come up with enough money to meet the often-required match funds when applying for loans and grants.
The irrigation district received some help in 2019 with funding the analyses of the dam to learn the extent of the damage. The Idaho International Laboratory donated $10,000 to the analysis costs, according to Pherson, and the irrigation district set up a savings account five years ago to collect funds for repairs.
Pherson said the irrigation district is waiting to hear back about hazard mitigation grants through the Federal Emergency Management Agency. Pherson said he has sought help from both the federal and state governments, but he said he has been met with dismissal and hesitancy.
“They tell me, and I’m the one that’s been doing all (the applying for grants), being that the feds or the state did not help build that dam, especially because the (Farm Service Agency) did not participate in that, then we are out in the cold,” Pherson said.
Out of 15 funding requests through U.S. Rep. Mike Simpson’s Community Project funding proposal to the U.S. House Committee, a grant for Mackay Dam was one of two that were denied. Both Idaho representatives later voted against the U.S. House of Representatives appropriation bill that could give funding to the 13 approved requests.
Lura Baker, Custer County clerk, said she was told the funding was denied because it was viewed as duplicate funding.
Flooding, spread of radioactivity and death are all risks
Should the dam fail, Mackay would be wiped out in six minutes, according to the complaint from the Environmental Defense Institute. Including Mackay, an estimated 600 people and their property in Leslie, Darlington, Moore, Arco and Butte City would be affected in the case of a dam failure.
“Initially, there can be a wall of water about 80 feet high going down from the dam,” McCoy said. “I believe it was those mountains there that created one of the greatest floods on the face of the earth.”
The dam is classified as a high hazard dam, Falk said, because in the event of failure, it would result in loss of life. However, Falk said the risk of spreading radiation pollution from flooding at the Idaho National Laboratory depends on the amount of water that reaches it. The dam itself is in fair condition, but the primary outlet and spillway are in poor condition, according to a 2021 dam inspection report.
“I certainly don’t envy the Big Lost River District,” Thatcher said. “These are just guys who wanted to, you know, irrigate and and they have a very difficult job of just managing that town, managing the routine maintenance, managing the day to day operations. That’s no job for cities.”