Idaho’s Mackay Dam needs repairs, from spillways to gates.


The view of May 12, 2021 looking upstream at the ten foot diameter riveted steel outlet conduit and adjacent manifold pipes. Reservoir discharge through the control tower was reported by Big Lost River Irrigation District staff to be 255 cubic feet per second that day, as documented in a department dam inspection record. of Idaho’s water resources in 2021.

Courtesy of the Idaho Department of Water Resources

This story was originally published on on August 15, 2022.

The over 100-year-old Mackay Dam in Custer County, Idaho needs repairs and poses a risk to the town of Mackay just downstream and the Idaho National Laboratory about 30 miles away, according to the Idaho Department of Water Resources and the Environmental Defense Institute, an Idaho nonprofit focused on nuclear energy issues.

The 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 dam’s age and lack of repairs have increased the risk of its failure in the event of a flood or 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 area, and the dam is near the Lost River Fault and Borah Peak.

From spillways to gates, the Mackay Dam needs repairs

The list of repairs needed for the structure is long, according to regular reports from the Department of Water Resources. The dam’s emergency spillway is too small and is deteriorating. The five gates of the dam are in various stages of decline and two cannot be used. A leak at the toe of the dam could contribute to increased erosion of the concrete of the dam itself, although the leak has not yet shown signs of major erosion. The dam has no warning system in the event of a dam break, human or electrical.

Central Idaho has a recent history of heavy rainfall, which has resulted in three Mackay Dam overflows in the past 15 years: in 2010, 2011 and again in 2017, according to a 2018 complaint to the Idaho government. ‘era. Butch otter. The overshoot event in 2017 came after warnings of heavy 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 high levels of snowmelt would fill the reservoir faster than it should. could not be emptied, thus increasing the erosion of the already damaged spillway. result of exceeding the maximum flow rates.

Spackman asked the irrigation district to increase the amount of water coming out of the dam. Due to a breach in a levee near Leslie, the Big Lost River Irrigation District refused to increase the amount of water coming out of the dam because it would cause flooding.

Regional earthquakes could cause Mackay Dam to rupture

Earthquakes also present a significant risk to the integrity of the dam as the seismic ratings of the construction are unknown. The Mackay Reservoir was built around 1921 by the Utah Construction Company, which helped build the Hoover Dam outside Las Vegas in 1931. The company built the Mackay Dam around the start of its construction days of roadblocks and did not have many precedents. experience with dams, according to the Environmental Defense Institute.

Concerns about the structural integrity of the dam and how it would hold up under 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,” said Dave McCoy, who previously worked for the Environmental Defense Institute and helped draft the complaint the organization filed in 2018.

Big Lost River Irrigation District Board Chairman Byron Pherson said that despite claims by the Environmental Defense Institute that the construction of the reservoir was not well documented, the irrigation district has plans and other documents detailing the construction of the dam.

According to the Environmental Defense Institute, according to the Environmental Defense Institute, seismic ratings for dams are not specified. Many regulations would be left to the discretion of the director of the Department of Water Resources, according to the draft new regulations.

In 2020, a magnitude 6.5 earthquake shook Custer County and a state of emergency was declared after minor foundation damage to some buildings in Custer County was observed, although the dam suffered no damage according to a letter of inspection from the Department of Water Resources. A resident told the Challis Messenger that the 2020 quake was reminiscent of the 1983 Borah earthquake which was 6.9 magnitude.

“The seismic analysis (of the Big Lost River Irrigation District) and understanding of the weir, even when it was built in the 1950s, is totally lacking and inadequate,” said Tami Thatcher, who writes about the nuclear issues for the Idaho National Laboratory and writing for the Environmental Defense Institute. “Then, after the Borah earthquake in 1983, the dam survived, but they were concerned about the rocky cliffs above the spillway. The following year, they hired someone to come and blast rocks.

Mackay Dam repairs won’t come cheap and funding has been hard to come by

Any repairs needed to make the dam more stable will cost millions of dollars, and Idaho Department of Water Resources Dam Safety Officer John Falk said the price will only increase.

Pherson said it’s difficult for small communities in the region to find enough money to meet the matching funds often required when applying for loans and grants.

The irrigation district received help in 2019 with the funding of analyzes of the dam to find out the extent of the damage. The Idaho International Laboratory donated $10,000 for testing costs, according to Pherson, and the irrigation district set up a savings account five years ago to raise funds for repairs.

Pherson said the irrigation district is waiting to hear about risk mitigation grants through the Federal Emergency Management Agency. Pherson said he sought help from the federal and state governments, but said he was fired and wavered.

“They tell me, and I did it all (the grant applications), since the federal government or the state didn’t help build this dam, mostly because the (Farm Service Agency) didn’t participate in it, so we’re in the cold,” Pherson said.

Of 15 requests for funding through U.S. Representative Mike Simpson’s Community Project Funding Proposal to the U.S. House Committee, a grant for the Mackay Dam was one of two denied. The two Idaho representatives then voted against the U.S. House of Representatives appropriations bill that could fund the 13 approved applications.

Lura Baker, Custer County Clerk, said she was told the funding was denied because it was considered duplicate funding.

Floods, the spread of radioactivity and death are all risks

If the dam failed, Mackay would be wiped out in six minutes, according to the Environmental Defense Institute complaint. Including Mackay, approximately 600 people and their properties in Leslie, Darlington, Moore, Arco and Butte City would be affected in the event of a dam failure.

“At the start, there may be a wall of water about 80 feet high coming down from the dam,” McCoy said. “I believe it was those mountains over there that created one of the greatest floods on the face of the earth.”

The dam is classified as a high-risk dam, Falk said, because if it failed, it would result in loss of life. However, Falk said the risk of spreading radioactive pollution from flooding at the Idaho National Laboratory depends on how much water reaches it. The dam itself is in good condition, but the main 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. “They’re just guys who wanted to, you know, irrigate and they have a really tough job running this town, running routine maintenance, running day-to-day operations. This is not a job for cities.