The construction of the Mackay Dam began in 1909. The 1894 Carey Act allowed private capital to be used in constructing the dam, with the stipulation that the state of Idaho would provide supervision of projects.

The state engineer took photographs and discussed problems with the construction company. Construction plans did not include a spillway needed during flood conditions. And in 1911, the state of Idaho built a spillway for the Mackay Dam, which served until the 1930s when the existing spillway was built.

A 1951 Bureau of Reclamation report stated that it considered the Mackay Dam an unsafe structure. It estimated the spillway capacity as only about 2,000 cubic feet per second. Since then, the storage capacity of the dam was increased and the concrete of the outlet control tower, tunnel and spillway have continued to degrade.

Meanwhile, the Atomic Energy Commission, later to become the Department of Energy, had selected several locations on the smooth soil of the Big Lost River floodplain at Idaho National Laboratory to build on and by 1952 was operating nuclear reactors and spent fuel reprocessing facilities.

Since that time, spent fuel reprocessing has ceased but the high-level waste produced — now in the form of calcine and liquid sodium-bearing waste — remains in the floodplain with no identified out-of-state disposal path. And spent nuclear fuel from various sources, including Three Mile Island fuel debris, is also stored at the former fuel reprocessing site now known as INTEC.

In 1983, the Borah Peak earthquake awakened interest in the safety of the Mackay Dam.

In 1986, the Department of Energy funded a study of the dam that concluded that the Mackay Dam, although it would not survive the probable maximum flood estimated for the dam and usually required of high hazard dams, would survive a 1,000-year flood.

Flood depths and time for the flood waters to reach INL were also estimated in the report, should the dam be breached. The 1986 study estimate of the spillway capacity was 6,588 cubic feet per second, a figure that has been cited by the Idaho Department of Water Resources.

The IDWR conducts inspections of state-regulated dams and issues the certificate for their continued operation.

In 2020, a study was underway for alternatives for Mackay Dam rehabilitation. Assessment of the current status of the dam estimated the spillway capacity of 2,200 cfs, far below the 1986 estimate. Then in 2021, the study found that the spillway degradation combined with the location of the spillway would be expected to fail not only the spillway but also the dam if spillway flows were sustained above 500 cfs.

This meant that not only would the dam not survive 1,000-year floods, it would not survive a 50-year flood under favorable conditions. The 95 percent confidence value was only about 25-year flooding to fail the dam.

And this is during a time that the U.S. is seeing unexpectedly high precipitation levels, like the 500-year flooding in Montana this spring, despite overall drought conditions.

Would you live in a house that on average was expected to collapse once every 25 years?

Failure of the Mackay Dam sends a wall of water far over 8 feet deep to the town and vacation area of the town of Mackay, and it has long been recognized that there isn’t time to evacuate people before the flood waters reach the town.

Funding for even modest upgrades has not been found. The Big Lost River Irrigation District struggles to pay for routine repairs.

And the failure of the dam will cost lives and also, at the very least, create enormous additional costs due to damage and difficulties created by the flooding at various INL nuclear facilities. Studies of the likelihood of a radiological release due to the flooding have not been released by the Department of Energy.

With the tendency for the Department of Energy to downplay any issue that might be problematic, and the tendency for state of Idaho agencies to keep their head in the sand, it appears that the plan is to take no action and wait for the inevitable catastrophic and very costly failure of the Mackay Dam.

Tami Thatcher is a former nuclear safety analyst at Idaho National Laboratory.