State orders demolition of LaPrele dam to avoid 'catastrophic failure'

It is no longer safe to hold any amount of water behind the LaPrele dam, and the long deteriorating 115-year-old concrete structure located 20 miles west of Douglas must be demolished as soon as possible to avoid a "catastrophic failure," according to state officials.

State Engineer Brandon Gebhart issued a " breach order " earlier this month, noting the recent discovery of several new cracks compounding other structural problems identified years ago.

"This dam has significant structural deficiencies and has exceeded its useful life," Gebhart said in a prepared statement.

In 2021, the Wyoming Water Development Office estimated that, at full storage, a catastrophic failure would send a torrent of water and debris through the Ayres Natural Bridge Park just two miles below the dam, overwhelm several roadway bridges and flood areas of Douglas along the North Platte River.

Fortunately, the state and local irrigation district had already taken measures to avoid such a scenario by draining the LaPrele reservoir earlier this fall.

"I want to commend the State Engineer and his staff for recognizing the significant risks of a potential dam failure and proactively addressing them before a disaster occurred," Gov. Mark Gordon said in a prepared statement. "This decision was not made lightly, and we recognize the impact this will have on those who rely on that water for irrigation."

Now that authorities have mitigated an immediate catastrophic failure of the LaPrele dam, the state is racing against the clock to demolish the structure before April when spring runoff begins.

"These [structural] threats need to be mitigated before the spring runoff, when flows are expected to exceed the dam's ability to pass inflows," Gebhart said.

Runoff is also a potential concern this winter, state officials added, noting there's always a possibility for unseasonable flows that might exceed the dam's ability to allow water to freely pass through the outlet, which has a maximum output of up to 300 cubic feet per second.

Local and state officials, in consultation with engineering firms, came to terms with the pending demise of the LaPrele dam several years ago and initiated planning for how to eventually replace it. Cost estimates reach above $118 million. In addition to a $30 million appropriation from the Legislature, the state has secured $5 million via the federal Infrastructure Investment and Jobs Act.

"There will be uncertainty on costs until the design is complete and a price can be agreed upon through a construction contract," Wyoming Water Development Office Director Jason Mead told WyoFile. "However, with the inflation seen over the last several years, it is expected additional funds may be needed to complete the project and everyone is working to identify potential sources for that funding."

After demolition work is completed, construction on a replacement dam may begin in 2026 with a completion date of 2029, according to Mead. The dam served about 100 irrigators along LaPrele Creek, which flows out of the Laramie Range into the North Platte River.

The dam, standing 130 feet high and stretching 325 feet over LaPrele Creek, serves late-season water to irrigators between the dam and the nearby North Platte River, according to the state. The Ambursen-style dam consists of a series of concrete buttresses — or "fins" — supporting an angled, flat slab on the reservoir side, and is anchored into a fractured Madison limestone formation on both sides.

In 2016, a boulder fell from the west wall on the dam's downstream side, barely missing the structure. The discovery of the boulder's near-miss led to investigations that revealed several migrating cracks in the dam, spawning further engineering investigations and a determination a few years later that the dam's days were numbered.

The dam's structural integrity, in fact, has been in question for decades.

Talk of LaPrele reaching the end of its useful life emerged in the 1970s due to leaks and the problematic nature of the Ambursen-buttress design. But the Panhandle Eastern Pipeline Co ., eager at the time to access water for its coal-gasification ambitions, struck a deal with irrigators to repair the structure in return for a share of water.

The company paid to grout cracks and add new layers of concrete. Panhandle Eastern Pipeline's coal-gasification plans never came to fruition, but the company's patching efforts restored confidence in the dam — for a while.

Irrigators reliant on the LaPrele dam have seen dwindling late-season water deliveries — by about half — since 2019, when the state ordered the reservoir be maintained at lower levels to avoid stress on the structure. Now those ranching operations rely completely on what Mother Nature delivers, without the benefit of measured releases for late-season crops or a functional dam to help avoid potential flooding.

"The farms and ranches will see significant production losses until the new dam is constructed," LaPrele Irrigation District Secretary Anna McClure said. "Streambank integrity and infrastructure on the LaPrele Creek will be affected by the uncontrolled flow of water."

State officials are assessing the flood risk, but so far there's no particular mitigation plan.

"There are areas downstream of LaPrele Dam that could be impacted by uncontrolled spring flows, including Ayres Natural Bridge," Gebhart said.