The estimated cost of a proposed reservoir above Hyattville in Big Horn County increased 69% to $59 million after studies revealed complex underlying geology and poor embankment material, among other factors.

As a result, lawmakers are asking for more money in the 2020 omnibus water construction bill now before the Legislature.

Engineers originally estimated the  8,000 acre-foot Alkali Creek Reservoir would cost $35 million, but that changed last year after geotechnical investigations revealed more detailed information. Now the project is expected to cost $59 million — an increase of $24 million over the original 2015 calculation.

The Select Water Committee’s 2020 construction bill seeks a $56.9 million grant for the project, up from $32.9 million approved by the Legislature in 2017.

Thirty-three irrigation shareholders would be on the hook for $2.1 million or 3.6% of actual development costs, whichever is less. The proposed 73% grant increase would reduce the watershed improvement district’s share of the project cost from 6% to 3.6%.

The Nowood Watershed Improvement District would own the dam and reservoir once built. They would hold water exclusively for use by Wyoming. The reservoir would serve 13,500 acres using flows from Paint Rock and Medicine Lodge creeks diverted to and impounded in the Alkali Creek drainage.

“I’m a little alarmed by the 70% increase in costs,” Rep. John Eklund, R-Cheyenne, said at a legislative meeting in November. “We’re using our savings account and at an alarming rate. The money we use to increase this then takes away from other projects.”

Eklund supported construction of the dam, however, calling it “a great project … from the get-go.”

Irrigators would pay $24 an acre-foot for their subscribed allocations annually, regardless of whether the reservoir fills or they use their water allowance. The price is based on what irrigators said they can afford to pay, according to discussion at the November meeting.

The dam would be 100 feet high, 2,600 feet long and inundate 294 acres of Bureau of Land Management and private property. Two additional embankments would help impound the reservoir.

The 2015 cost estimate was “pretty coarse” Jason Mead, Water Development Office deputy director for dams and reservoirs told the committee in November. During final geotechnical studies, engineers discovered the geologic structure under and around the proposed dam “was much more complex” than originally believed, he said.

Seepage was to be cut off with a bentonite/clay blanket, he told the committee. “Now we’ve realized we’re going to have to do some grouting,” a process in which cement is injected into subterranean fissures.

Further, the earth with which the dam is to be built turned out to be “significantly weaker than what we thought,” he said. Consequently, more earth will be needed to make a dam with gentler slopes than originally conceived.

The dam’s shell will be gravel, not clay, and haul distances for materials will be longer.

Nevertheless, the project would be an economic benefit, based on a 50-year lifespan, Mead said. The facility could operate for 75 years, he said.

He pegged benefits at $133.7 million — a 2.27 benefit/cost ratio. Public benefit — derived from a 2,000-acre foot reserve pool that would hold fish, plus public access to a boat ramp — amount to about $92 million, he said.

Uncounted benefits accrue to downstream wetlands, riparian areas, fisheries and from “keeping families in business over time,” Mead said. While only 33 shareholders have signed up, they represent others. “A ranch has many folks they employ,” he said.

The project is important for the area around Hyattville, John Joyce, chairman of the Nowood Watershed Improvement District told the committee. “We’re putting Wyoming water to beneficial use — it’s not going to Montana any more,” he said.

The reservoir is designed to provide late-season irrigation to farmers and ranchers who otherwise rely mainly on snowmelt for their water supply. Joyce expects the project to receive a U.S. Army Corps of Engineer’s permit next summer, he said.

The BLM authorized the plan after completing an environmental study. The agency rejected an alternative that would have reduced the project’s footprint by constructing a smaller inflow ditch and reducing the length of an auxiliary spillway.

“Greater sage-grouse use of the project area may be reduced,” the BLM said in approving the project. The reservoir would flood more than 2 miles of Alkali Creek, considered an intermittent stream. As such, the reservoir is considered an off-channel impoundment that doesn’t inundate either Paint Rock or Medicine Lodge creeks and their riparian environments.

The geologic discoveries, however, will also hike the cost of another nearby reservoir project, increasing the estimated cost of the Leavitt Reservoir expansion by $5 million. Instead of $41 million, it is now estimated to cost $46 million.

A grant to fund that project would increase 12% from the previously approved $39.3 million to $44.3 million. A loan to irrigators would remain unchanged at $1.7 million.

Eklund said the state was spending “more on regulations and garbage than construction,” at the Alkali project outside of Hyattville. But Mead said the environmental compliance studies account for about half the $4 million permitting and design costs.

The Alkali project started in the fall of 2007 and design will be finished in 2021, Mead told the group. The impoundment should begin filling in 2023 and 2024, he said.

The Legislature set up a fund of severance tax revenue automatically diverted toward water development that began accruing approximately $775,000 annually in 2005. Lawmakers have supplemented the account with additional funds.

There’s about $63 million in the construction account today — enough to complete the Alkali Reservoir, Mead told lawmakers. The Legislature, nevertheless, must approve the construction bill that includes the proposed changes.

All told, the 2020 omnibus water construction bill would appropriate $87 million in new money to the Wyoming Water Development Office for additional Alkali Reservoir and Leavitt Reservoir funds, plus other projects.

(WyoFile is an independent nonprofit news organization focused on Wyoming people, places and policy.)