State orders breach of the failing LaPrele Dam

Gov. Mark Gordon issued a press release on Nov. 1, stating following ongoing monitoring and a recent thorough inspection of the condition of LaPrele Dam, Wyoming State Engineer Brandon Gebhart issued an order to breach the 115-year-old dam to avoid catastrophic failure.

According to the governor’s press release, the recent discovery of a new large crack above the dam’s outlet works, the deterioration of previously identified structural deficiencies and the operational limitations of the existing dam led to the decision.

“I want to commend Gebhart and his staff for recognizing the significant risks of a potential dam failure and proactively addressing them before a disaster occurred,” Gordon says. “This decision was not made lightly, and we recognize the impact this will have on those who rely on this water for irrigation.”

However, farmers who rely on the dam for irrigation and runoff control say this breach could leave them with no good options.

The dam serves roughly 100 irrigators, but it will be more than $118 million to replace and will be demolished before the spring runoff season, according to recent reports.

The LaPrele Dam

Currently, the LaPrele Dam is owned and operated by the LaPrele Irrigation District and is located on LaPrele Creek southwest of Douglas.

The 135-foot-high LaPrele Dam stretches over 325 feet and was constructed between 1906-09 by the Ambursen Hydraulic Construction Company of Boston to provide irrigation water under the Carey Act of 1894.

The Ambursen-style dam consists of a series of concrete walls, or fins, to support an angled, flat slab on the reservoir side and is anchored into a fractured Madison limestone formation on both sides.

Built to have a 50-year lifespan, the dam began to show damage in the 1970s due to leaks and the problematic nature of its design.

According to documentation, the Panhandle Eastern Pipe Line Company struck a deal at the time to repair the structure in return for a share of the water.

The company paid to grout cracks and add new layers of concrete, but the patching only restored confidence in the dam for a short time.

After a boulder fell from the west wall on the dam’s downstream side in 2016, an investigation revealed several migrating cracks in the dam, initiating further engineering investigations. 

Since 2019, the state ordered the reservoir be maintained at lower levels to avoid stress on the structure.

Given the age and condition of the structure, LaPrele Dam was already scheduled for replacement in coming years.  

In October, engineers found a new large crack above the dam’s outlet works and discovered the older cracks were worsening.

Next steps

Planning for the replacement is underway, and these plans include demolition of the dam which was set to occur as part of the replacement project.

“The breach order now requires the demolition to be completed prior to the upcoming spring runoff season,” notes the press release.

“The deteriorating conditions of the dam have been identified and monitored by the State Engineer’s Office, the district and the Wyoming Water Development Office,” the press release further notes. “Currently there is little to no water behind the dam, and the dam’s outlet works are completely open and passing all inflows into LaPrele Creek.”

Gebhart states, “This dam has significant structural deficiencies and has exceeded its useful life. Due to preemptive actions already taken, there is currently no risk or danger to the public downstream because there is little to no water stored, and the outlets are able to pass inflows to prevent new storage.”

“However, continued use of the dam to store water could cause catastrophic failure, which poses a threat to life and property,” he continues. “These threats need to be mitigated before the spring runoff, when flows are expected to exceed the dam’s ability to pass inflows.”

Breaching the dam could leave downstream landowners wide open to catastrophic spring runoff.

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

State officials note runoff is also a potential concern this winter, noting there’s always a possibility for unseasonable flows which may 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.

According to government officials, consulting with engineering firms began several years ago to initiate planning on how to eventually replace the LaPrele Dam.

Cost estimates reach above $118 million, in addition to a $30 million appropriation from the legislature.

The state has secured $32 million via the federal Infrastructure Investment and Jobs Act and hopes to secure the rest of the necessary funds from the same federal program.

Potential challenges

Recently, at the Wyoming Legislature’s Select Water Committee meeting held on Nov. 13-14 at Casper College, landowners downstream of the dam gathered to express their concern for the spring runoff.

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

Local residents impacted by the breach of the dam spoke during the public testimony section of the committee meeting stating their concerns and provided heartfelt statements. 

Shane Cross, a member of the Boot Ranch west of Douglas and a shareholder of the LaPrele Irrigation District, delivered comments on how the breach would impact the local economy and community.  

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

To hear remarks made regarding the LaPrele Dam during the committee meeting, visit youtube.com/live/HOYUpwRJGNs.

Melissa Anderson is the editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.