The Wyoming Water Development Commission organized a tour of the LaPrele dam Aug. 12, 2021. Constructed in 1909, the dam is now considered at risk of catastrophic failure. (Dustin Bleizeffer/WyoFile)

DOUGLAS — A pair of engineering firms contracted by the state warn that cracks in the 112-year-old concrete LaPrele dam, along with deterioration in its geologic foundation, could result in catastrophic failure. 

Such an event would threaten people and infrastructure downstream, including the town of Douglas, and likely destroy the Ayres Natural Bridge Park directly below the dam, as well as two Interstate 25 bridges, the Wyoming Water Development office said during a tour last week.

“This is more than an irrigation district matter,” Water Development Office Director Brandon Gebhart said. “It’s a hazard.”

Rehabilitating the dam might be impossible, according to Gebhart. Instead, the office and its governing citizen commission is considering building a new dam directly below the existing structure. The commission will consider seeking funds to develop a complete engineering study for a project that could exceed $50 million.

LaPrele Reservoir has been maintained at a lower level since 2019 to help relieve pressure on the compromised dam. (Dustin Bleizeffer/WyoFile)

Such an effort may qualify, in part, under several provisions in the $1 trillion infrastructure bill passed by the U.S. Senate this month, buttressing a Wyoming policy ambition to impound more water for use before it leaves the headwaters state, according to state officials. Meantime, the state is examining multiple other funding sources due to the potential for loss of life and property if the dam were to fail.

The matter is urgent, officials say.

“It’s not a slow failure with this dam design,” Gebhart said. “It would be very abrupt.”

The water office organized a public tour of the dam Aug. 12 to better inform the public of the risks and potential solutions under consideration.

A rumble and a warning

When a magnitude 3.7 earthquake rumbled the towns of Glenrock and Casper on the night of Aug. 1, personnel at the nearby Ayres Natural Bridge Park said it felt like a sonic boom. They sprang into action, caretaker Dee McDonald said.

Dee and her husband Doug McDonald have served as caretakers for six years.

The natural arch was still intact. But their minds were on a more pressing danger still looming: water. So they evacuated the handful of campers in the park that night.

“Anybody with any sense would do that,” McDonald said.

The Ayres Natural Bridge is located less than two miles downstream from the LaPrele dam, which engineers warn could suffer catastrophic failure. The popular swimming hole at Ayres Natural Bridge was closed after a summer rainstorm knocked loose some rocks from the structure. (Dustin Bleizeffer/WyoFile)

The park is situated in a small geologic bowl filled with old boxelder and tall cottonwood trees, along with manicured grass and picnic areas surrounded by high sandstone cliffs carved out over millennia by LaPrele Creek. The creek runs directly under the natural bridge. The rare high-plains oasis was a retreat for European settlers traveling the Oregon Trail. Today it remains a popular leisure destination for locals and travelers alike.

Visitors from around the world packed into the park on Aug. 21, 2017 to experience the totality of the solar eclipse that swept across central Wyoming.

Less than two miles upstream, however, is a not-so-natural wonder: the LaPrele dam. Completed in 1909, the concrete structure today presents a danger to an area far beyond the Ayres Natural Bridge Park, state officials and engineering teams say. 

Aging, patchwork infrastructure 

The Ambursen-style-designed LaPrele dam consists of a series of concrete buttresses supporting an angled, flat slab on the reservoir side. It is 130 feet high and 325 feet wide, and serves late-season water to about 100 irrigators via 94 miles of irrigation infrastructure, according to the state. The dam is anchored into a fractured Madison limestone formation on both sides.

Construction was funded via the federal Carey Act of 1894, a measure pushed by Wyoming U.S. Sens. Francis E. Warren and Joseph M. Carey to help arid western states develop more water for irrigation.

The Ambursen-style design LaPrele dam consists of a series of concrete walls — or “fins” — to support an angled, flat slab on the reservoir side. The design is prone to catastrophic failure, according to engineers. (Dustin Bleizeffer/WyoFile)

In the 1970s, the LaPrele dam was determined to have reached the end of its useful life. But dozens of irrigators downstream still depended on late-season releases from the reservoir to help them eke out a living on the plains along the North Platte. Neither the state nor federal government were eager to pay for refurbishing the dam, so the Panhandle Eastern Pipeline Co. agreed to fund the repair effort in return for a share of water for a coal-gasification project.

Crews grouted cracks and added new layers of concrete to the dam. Panhandle Eastern Pipeline Co., along with the lead engineering company that oversaw the project, received the “Outstanding Civil Engineering Achievement of 1979” award from the American Society of Civil Engineers.

Panhandle Eastern’s coal gasification project never came to fruition. Grateful for the investment and newfound confidence in the dam, however, members of the LaPrele Irrigation District went about their business of relying on the patched up dam for late-season water.

Then, in 2016, a boulder in the west limestone wall behind the dam fell. The event was listed as a mere notation in records reviewed a couple years later by a team commissioned to assess water and water infrastructure along the North Platte.

Discovered by accident

When the team of engineers examined the rockfall behind the LaPrele dam, it alarmed them. 

Peter Rausch of the engineering firm RESPEC said that during the initial inspection it appeared that if another large boulder in the same strata were to fall it might roll into one or more of the dam’s concrete fins. Then, looking at the potential targets, they noticed what looked like a large, unrepaired crack in one of the boulder-zone concrete structures.

A boulder in the limestone wall behind the LaPrele dam fell in 2016. Engineers warn that if rock in the same strata were to fall, it could damage concrete structures in the dam that are already compromised with unmitigated cracks. (Dustin Bleizeffer/WyoFile)

RESPEC called another firm, HDR, which specializes in dams, to assess the structure itself. Apart from the potential of damage via rockfall, developing cracks in the dam pose a risk of catastrophic failure, according to the company.

In November 2019, the Wyoming Water Development Office ordered the LaPrele Reservoir be maintained at a lower level to avoid stress on the dam. Consequently, the LaPrele Irrigation District receives about 55% of its normal appropriation of water, according to the state.

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That’s concerning enough for eastern plains irrigators facing a warming, drier Wyoming where late-season irrigation is becoming increasingly vital in the face of a global climate crisis. But the risk of a dam failure, and the catastrophes that might result, add to anxieties — even if it might move a new dam construction project higher up the list of infrastructure priorities.

“This is a classic example of aging infrastructure,” Gebhart said. “We were lucky to find this — it was happenstance that we found the deficiencies in this. 

“I think there’s probably a lot of infrastructure in the state, not just irrigation but also municipal, that is maybe at or near the end of its useful life,” Gebhart said. “Some of it may not even be known. It’s an example of a significant problem I see in the state.”

Dustin Bleizeffer is a Report for America Corps member covering energy and climate at WyoFile. He has worked as a coal miner, an oilfield mechanic, and for 25 years as a statewide reporter and editor primarily...

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    Over the years I’ve been amazed at the sheer number of water development projects which have made living in most of Wyoming possible. There are literally 10s of thousands improvements spread across the state like a web. Included are thousands of small stock dams, thousands of windmills, large irrigation reservoirs, large canals and smaller ditches and spreader ditches, headgates, buried water pipelines by the thousands of miles, storage tanks, domestic and livestock wells, large capacity ground water irrigation wells, artesian wells, sink holes that recharge ground water, beaver dams, lakes, sloughs, wetlands, municipal water systems and treatment plants, etc. The point is that there has been a concerted effort throughout the history of Wyoming to develop the water resources making much of Wyoming habitable – and this has been public policy from Washington on down. Included are the State of Wyoming statutes that govern water in Wyoming. This is no small deal – this whole system of water development is MEGA important. The LaPrele Dam is an important component of the States water resources and should be maintained as long as possible despite the costs.

  2. Wyoming, unfortunately, has always been behind the times in more ways than one. The maintenance of this damn is a good example. $50m seems small compared to the cost of replacing parts of I-25 and the loss of the town of Douglas and the lives therein. I’ve lived here for 41 years and love this state, and I’ve spent alot of time at Natural Bridge through the years, and frankly, I thought the damn was in desperate need of repairing over 20 years ago.

  3. Huge conversation starter and that’s my 2 cents since it seems there is a lot to discuss, and apparently, quickly. Point about benefit to water users, current and future, is well made. Who takes the lead? Wyo political culture seems to argue that ineptness in state government is not first option, although it is the entity that should step up given the likelihood it will be on hook for the cleanup if dam does what scientists are most worried about. Impressive journalism in my view. Hope the response measures up.

  4. This article missed the most important discussion point. That would be the cost and benefit of replacing the dam. There seems to be an assumption that the State and Federal government (taxpayers) should be part of the financial solution to help sustain private business. The questions becomes is the $50M cost of replacing the dam worth cost/benefit to the public? Keep in mind that cost does not include the interstate bridges and highway. Exactly how much economic benefit is the irrigated land to US? It will take a long time (realistically never) for the irrigators to pay enough taxes to cover the $50 M, plus interest. If the answer is its little or no overall benefit, the irrigators should pay for the entire cost. We all know that’s not gonna happen. There is a reason neither the State or Feds were not interested in participating in the project. It could probably be summed up in “not worth the cost” to the public. Some aging infrastructure should just be removed, This appears to be one.

    1. Excellent point. I still struggle with the benefit I get from my tax dollars going to these projects. I would love to have the ag folks enlightenment me. Seriously. I’m not in the business and maybe there is something I’m missing. If I realize a benefit from my tax dollars going to fix the dam then I’m fine with it. Otherwise let those who benefit own the problem.

  5. Thanks for a well-written article. And kudos to the engineers who did the studies.
    An excellent book about a dam failure is The Jonestown Flood by David McCullough. It’s a tragic story about a dam that was built for one purpose, but over time the maintenance and upkeep was passed to others or forgotten. Small changes ultimately led to it failing and nearly wiping out the town of Jonestown. It’s a compelling lesson that resonates now, particularly with our aging infrastructure. Civil engineering structures need to have continuous maintenance, and like our highway system (built in the 1950’s to 1960’s), many are near the end of their intended design life. Water and sewer systems in particular need to be maintained. Consider that a change in the water supply in Flint MI and failing to add anti-corrosive compounds to the water treatment caused the lead disaster in the water system. We must continue to maintain and upgrade our civil engineering structures, and ensure we have the proper professionals involved in their maintenance and operation.

  6. Champlain Towers South condo building collapse ring a bell? They knew all about its problems well ahead of time, too.

    Trillions of dollars. And over one million people involved in nation building in foreign countries. Productivity (?), energy, money, and time that could have been used, should have, to rebuild America.

  7. Great information, solve the problem before it becomes even more costly . It will be more cost effective to build or redo this dam soon rather than pay financially and emotionally the cost of a disaster to everyone up stream and down stream, in the near future.

  8. Well written and informative, from the area and camped above the dam .Build a new dam , headwaters are found gold , in 50 years it will be actually priceless. The new dam construction will provide good paying jobs and economic opportunities for the entire county. Build a new dam!

  9. Shannon Anderson the dam was built in 1894 for irrigation purposes. Not for anything to do with coal, according to this article. I’ll have to add this to the “150 years of trying to grow crops where nothing grows and domesticated animals come to die” saga I am currently working on.

    1. Actually it was built for hydro-electric purposes, but not consistent enough water to make enough to continue funding it. It’s privately owned by the LaPrelle water district I believe.

  10. How about using $50,000,000 or less to buy out or compensate downstream users and get rid of the dam?

  11. This is a well-written, informative article with no hidden agenda. Thank-you. I believe this is what real journalism should be.

    1. 100% agree. Can you imagine what we could accomplish with straight forward fact based journalism that has no agenda. Thank you.

  12. We know a lot more now about dams and watersheds that we did 100 years ago. A more sustainable solution might be a smaller dam for flood control and getting the irrigation users to start rethinking their growing strategies. This is an issue all over the west. We are one of the only world powers that allows foreign countries (think China and Saudi Arabia) to buy land and water rights to grow hay and alfalfa for export, drawing down water tables. Not saying that’s the issue here, but it’s another one that needs to be addressed. Our uses of water over the last 100 years have been reckless and we’ve known that for decades.

    1. Wow I did not know that other countries owned American farming soil…..thanks for all the info concerning the dam. I do know that Utah is experiencing some of these same issues as it pertains to irrigation.

  13. Disturbing but important information for those living or working below the dam. Astonishing we’ve let maintenance of the dam lapse for so long!