If the LaPrele dam in Converse County were to suddenly give way, the resulting deluge could threaten downstream homes, rip through a key bridge along Interstate 25 and destroy the Ayers Natural Bridge Park.
It’s a possibility that lingers on the minds of the Converse County Commissioners, commissioner Robert Short told Cowboy State Daily.
“We’ve done tabletop exercises, our emergency management is well aware of how serious it could be,” he said.
Remote monitoring devices along the LaPrele Creek drainage below the dam – which is southwest of Douglas – would trigger an alert system if the dam broke, Short said. That in turn would send a warning message to the cellphones and other electronic devices of people in harm’s way.
“It would work much like an Amber Alert,” Short said. “It would send out the message that there was imminent danger, and to get out, now.”
Since 2019, LaPrele Reservoir has been kept well below its capacity to minimize risk to the aging dam, Converse County Commission Chairman Jim Wilcox told Cowboy State Daily.
“We’re not really worried about failure, because they’re monitoring the dam,” he said. “We’re more concerned about not being able to completely fill the reservoir in order to serve the irrigators who depend upon it.”
The dam was built in 1909, and inspections by engineers in recent years have revealed cracks and other structural flaws, Short said.
“It was originally scheduled for a 50-year service life,” he said. “It’s the last dam of its kind, and I believe the one at the highest elevation and the farthest north of the irrigation dams that were built at that time.”
The dam’s flaws prompted the order restricting the reservoir’s capacity, Wyoming State Engineer Brandon Gebhart told Cowboy State Daily. He heads up the state’s dam safety program.
The LaPrele dam’s structural weaknesses “are a matter of age,” he said.
The reservoir’s level is measured according to the elevation at the water’s surface, he said. The reservoir’s level can’t exceed 5,475 feet under the restrictions. That’s about 60% capacity.
At full capacity, the reservoir could hold roughly 20,000 acre feet of water, Short said. An acre foot is the amount of water it would take to cover an acre of land a foot deep.
About 11,462 acres of agricultural land, owned by about 100 users, depend upon the reservoir for irrigation, Short said.
The dam was built in the “flying buttress” style, which was popular at the time. “That means concrete buttresses protrude from the front of the dam,” he said.
Modern dams have a smooth front designed in an arch, Gebhart said.
“At the time those (dams like the LaPrele) were built, the buttress design was a result of cheap labor and expensive material,” he said. “Building a dam in that style was more labor-intensive, but it took less material to do so.”
Replacement Likely, Too Costly For County
The LaPrele dam will have to be refurbished or replaced – probably replaced – sooner rather than later, Gebhart said. No date has been set for when such a project could begin.
The price tag could soar as high as $80 million, well beyond what Converse County could afford, Short said.
The Wyoming Water Development Commission has approached the Legislature for funding, Gebhart said. There might also be federal infrastructure improvement funds available.
“It’s a high-priority project for us,” he said.