Consultants examined other sites for a new reservoir near New Fork Lake and determined that lowering the lake’s outlet and rehabilitating the existing dam would be the best alternative for providing additional water storage for irrigators. Although a study documented water savings through conservation, the Water Development Office doesn’t significantly fund such improvements and so is further studying the $12.7 million reconstruction and lower-outlet plan. (Wyoming Water Development Office)

A long-awaited study on how to increase water storage in Sublette County’s New Fork drainage recommends a plan that would drain the lake an additional 7.1 feet.

The study, which has cost Wyoming $296,000, recommends spending an additional $400,000 for further investigation of a $12.7 million project to rebuild New Fork Lake dam. Consultants for the Wyoming Water Development Office say the effort would aid 94 irrigators and provide other incidental benefits.

Reconstruction of the aging dam at the lake’s outlet, which is only a few feet high, plus a new drain 7.1 feet below current low-water level, is technically feasible and likely permittable, the study says. “I don’t think anybody’s seen fatal flaws at this point,” said Jason Mead, director of the Water Development Office’s department of dams and reservoirs,

New Fork Lake lies in the Bridger-Teton National Forest on the western edge of the Wind River Mountains. Maps show its edge as within 400 feet of the Bridger Wilderness. Forest Service approval would be required for a new storage project there.

The development, which would be paid for by some as-yet-undetermined combination of public grants and low-interest, long-term loans, would not increase the amount of water held in the New Fork system. But it would increase the amount of water that is available to be drained from the lake for irrigation. This, in the parlance of water development, is synonymous with “storage.”

Irrigators benefiting from the $12.7 million project might have to pay as much as 35 percent of the cost or as little as 10 percent through low-interest loans, according to scenarios sketched out in the study. The balance would likely be provided by the state, which could also finance any loans.

The additional irrigation flows would support 530 additional cattle for 94 private irrigators. The overall area agricultural benefit would be $496,000 per year, the study said. Additional incidental benefits, from improving safety to aiding wildlife and fisheries, also would result, according to the study. Adding storage for late-season irrigation would improve the habitat for fish.

New Fork Lake is 150 feet deep near the existing dam and a steep underwater drop-off would enable construction of a lower, controlled outlet.

Existing dam needs work

The existing dam is aging and inappropriately acts as a bridge to reach recreation sites on the lake.

The existing New Fork Lake dam is old and in need of rehabilitation. (Angus M. Thuermer Jr./WyoFile)

The New Fork Irrigation District served by the dam has an 8 percent average annual irrigation shortage, amounting to 6,700 acre feet, according to the study. That’s about enough to fill 3,350 Olympic swimming pools. The proposed solution would reduce, but not eliminate the shortage.

The Water Development Office asked RJH Consultants Inc. to name the top three water storage alternatives, and to also investigate water conservation. Conservation would include lining ditches and converting from flood to sprinkler irrigation. “Average annual available flow at New Fork Lake could increase from 11,200 to 24,500 acre-feet with application and conveyance efficiency improvements” — ditch lining and sprinklers — a hydrology appendix by Wenck Associates, Inc. says.

The conservation figures come from a model based on a theory of irrigators diverting 1 cubic foot per second of water for every 70 acres, as set in Wyoming water law, Mead said. In that model, “with conservation, you’ve got more water than with the enlargement.”

But the Water Development Commission isn’t in the business of lining canals and converting flood operations to sprinklers. “This project was focused,” said Andrew Linch, project manager at the Water Development Office. “It’s a storage project — not [a study to] look at the economics of application efficiency.”

The report’s recommendation to pursue dam rehabilitation and a lower outlet recognizes the agency’s assessment of what is practical, Mead said.

“There’s many miles of ditches in the New Fork Irrigation District,” Mead said.  “Piping [or lining] all of the ditches — you would improve efficiency,[but] the likelihood [is] that would be a very large undertaking.

“It’s cost-prohibitive, at least through our program, to implement that,” Mead said. Lining ditches and financing sprinkler irrigation would have to be accomplished through some other entity. Thus, the Water Development Office is promoting more storage.

“It really comes back to the economic [feasibility] to get [water] on the ground and that’s in our program – storage,” he said.

A federal permit would be required

The New Fork project would require a special use permit from the Bridger-Teton National Forest. A federal permit would require examinations of alternatives to using National Forest Land for a new storage or impoundment, which the study so far has done. Of 12 reservoir sites considered, eight were off the Bridger-Teton National Forest, but none made the top 3 alternatives, which focus on New Fork Lake.

Linch isn’t certain the Forest Service will compare storage against irrigation efficiency. “I don’t know if conservation would fit in [the parameters of] their special-use permit,” he said.

This map shows the New Fork Lake Irrigation District and nearby rivers, streams and lakes north of Pinedale in Sublette County. The Wyoming Water Development Office is pursuing a $12.7 million project that would increase irrigation water by adding a controlled outlet 7.1 feet below the existing low-water level at New Fork Lake. (Wyoming Water Development Office)

Mead said he believes a federal environmental study would likely broach the topic. “I’m sure it will be discussed,” he said of conservation. “Those questions will be asked and some more information might need to be put together.” Meantime, the Water Development Office will proceed with more technical studies.

At the Bridger-Teton, Pinedale District Ranger Rob Hoelscher said the alternative conservation issue “should be covered” in an environmental assessment or impact statement. “You would want to consider all those other things, like efficiencies, so you could compare the impacts of the project on that wide range of alternatives,” he said.

Consultants and the water Development Office believe the plan could secure a federal permit, based in part on past project experience, Mead said. To consider an option like conservation, that option must be available and practical, he said. For the Water Development Office, that’s a stretch.

“If there’s no way to actually implement it, is it available or practicable?” he asked. “Probably not.”

Incidental benefits of flood irrigation

There’s another reason to “store” more water in New Fork Lake by rebuilding the dam and lowering the inlet, supporters say. Many flood irrigators turn out two cfs per 70 acres, not the one per 70 anticipated in state law, Mead said. The excess is called “return” flow. Such water is said to return to the river either directly or through the water table and recharged spring creeks and to provide other environmental benefits.

“You put that water on the crop, it goes back to the stream, the next guy uses it again,” Linch said. “It ends up being used multiple times.”

Such water “ends up making the stream live or having water longer,” Mead said, “extending the hydrologic period in a given year.”

With return flows “you’re actually keeping the stream alive longer,” Mead said. Without impoundments and flood irrigation, “the water would run down the river and you wouldn’t have all the return flows in July, August, September.”

The New Fork River is a blue-ribbon trout stream that attracts anglers and their guides from around the region, if not the world. Wildlife also benefits from healthy riparian systems and agricultural land, Mead and Linch said.

“I can’t see where there’s a conservation downside to that project,” said Albert Sommers, a state representative and rancher who irrigates his Sublette County ranch with water from the nearby Green River. At New Fork Lake, there could be associated impacts to boaters if the lake was drained 7.1 feet below the low water line. But downstream, Sommers sees only benefits.

Albert Sommers, who irrigates his ranch with water from the Green River, says flood irrigation is a form of conservation. It reuses water, raises the water table and recharges springs. Overall, flood irrigation benefits wildlife as well as ranchers. “It’s well documented flood irrigation is what has allowed for late-season in-stream flow in the New Fork,” the state legislator said. (Angus M. Thuermer Jr./WyoFile)

“For many decades, there’s been this fear of dams,” he said. “I think we need to rethink some of that. We are losing glaciers,” which provide late-season flow as they melt. “Where are you going to store that water for fish, wildlife, agriculture?” he asked.

It’s well documented flood irrigation is what has allowed for late-season in-stream flow in the New Fork,” he said. Sommers likened agricultural fields to a sponge.

“In a time that typically we have seen less and less water in the system, the ability to store that water for all uses — I think that’s a good thing,” he said. “This doesn’t seem that impactful,” he said of the plan. “If you can store water in a time we have seen fairly historic drops in water volumes, I think that is conservation of water.”

The alternative, in his view, would be to “just let it flow south.” In such an instance “are we conserving water for California?” he asked. “If the importance is that California needs more water, then absolutely we should dry Wyoming up and send it down the river.

“I really do believe there is significant value in the way we irrigate today,” Sommers said. “By putting in these [flood] irrigation systems, you can make water go further. I think we’re in a better position if we can show beneficial use of our water, including recreation.”

The study lists additional benefits accruing for recreation, fisheries, public safety and fire suppression. More storage also would bolster the area’s economy, make ranches and their open space more viable, and be a boost to the community, Mead said.

Who would pay?

The study sees the project as economically beneficial, with a 1.24 to 1 benefit to cost ratio — without accounting for the economic boost that would accompany construction. Funding for the project has yet to be determined. The irrigation district would likely apply for a grant through the water development program “to cover the majority of project costs,” the study summary says. “The remainder of the capital costs could be covered through a [Wyoming Water Development Commission] loan to the District to be paid off over a period of 50 years at an interest rate of 4 percent.”

The study examines grant funding at levels of 65, 75 and 90 percent. “The current assessment [to irrigators] of $1.50 per acre would need to be increased many times over to cover the total anticipated annual loan costs,” the summary says.

Irrigators are willing to pay more, Mead said. They’ve got very minimal assessments at this point,” he said. Around Wyoming, irrigators will pay annual fees of from $5 to $15 per acre. Irrigators benefitting from an expansion of the Upper Leavitt Reservoir in the Bighorn Basin, for example, are willing to pay up to $25 per acre, he said.

A grant covering 67 percent of the New Fork cost — the least amount of state aid considered — would result in per-acre assessments rising to $14.04, an 836 percent increase. With a larger grant from the state — one covering 90 percent of the $12.7 million project — irrigators would pay $4.73 an acre every year.

“Overall, that’s pretty economical and reasonable, at least from our experience,” Mead said. “It’s one of the most economical [projects] in terms of a cost of basic acre-foot.”

New Fork Lake provides recreation for boaters, as seen in this 2016 photo, and feeds a blue-ribbon trout fishery downstream on the New Fork River. More of the lakebed would be uncovered and a larger shoreline “bathtub ring” exposed if the lake’s outlet was lowered 7.1 feet as planned. (Angus M. Thuermer Jr./WyoFile)

Linch said discussions with New Fork Irrigation District board indicate an assessment increase “would be feasible to consider. That’s why we’re continuing to the next phase of the study.”

In addition to the top choice of rebuilding the dam and lowering the outlet, RJH Consultants Inc. also considered building a new, taller dam. The third-best option for increasing storage would be a combination of a taller dam and lower outlet.

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Building a taller dam — a plan with associated consequences to wetlands and shoreline — would cost an estimated $25.4 million. The combination plan to raise the dam a bit and lower the outlet would cost $14.1 million. Numerous reservoir sites on and off the Bridger-Teton also were considered but none was among the top three storage choices RJH was asked to list.

The New Fork Irrigation District originally requested the existing dam be raised three or four feet. But the additional storage gained from that would not make sense when compared to a project to rebuild the dam and lower the outlet 7.1 feet, Mead said. “They didn’t have hard numbers,” he said. “In order to get them what they really wanted, we had to go a bit bigger.”

Technical numbers

Rebuilding the dam and lowering the outlet would provide a 9,400-acre-foot enlargement of New Fork Lake storage but a “firm yield” of only 3,000 acre feet in 8 of 10 years, the study says.

Farther downstream, the proposed enlargement would deliver an additional 2,400 acre-feet to irrigation headgates, “taking care of a little over one third of the shortages — one third to 40 percent,” Mead said.  

The RJH study was originally due last September but completed and made public just last month. Meantime, the Wyoming Legislature budgeted another $400,000 for the next phase of investigation, which includes geotechnical work.

The hydrology appendix assumed canals have an 80 percent conveyance efficiency and flood irrigation — used in the district — has a 55 percent application efficiency.

Other WyoFile stories on Wyoming dam construction and funding:

Four dam projects would cost $88M — Jan. 17, 2017

State moves on New Fork Lake dam — June 2, 2015

Wyoming dam construction plans advance — Dec 9, 2014

This article was corrected July 17 to reflect the amount of water an irrigator can divert. The flow is measured in cubic feet per second, not acre feet. Jason Mead provided a technical explanation on the diversions allowed below — Ed.

Wyoming Water Law allows the 2nd cfs based on the following:

Water rights for irrigation are adjudicated on the basis of 1 cubic foot per second (cfs) per 70 acres.  Irrigation rights with priority dates of March 1, 1945, or earlier are entitled to an additional 1 cfs per 70 acres. Those individuals who hold such a water right are entitled to divert water in the volume of 2 cfs for each 70 acres of land before any water is made available to the holder of a water right with a priority date after March 1, 1945.

If there is not sufficient water to furnish 2 cfs to each individual with a pre-March 1, 1945, water right but there is more than enough to furnish 1 cfs to each person, the surplus water is divided on a pro-rata basis. If there is so little water that each holder of a pre-March 1, 1945, water right cannot receive 1 cfs, the water is regulated on a strict priority basis.

Any water beyond that required to furnish 2 cfs for each 70 acres of a pre-March 1, 1945, water right is first allocated to rights with priority dates after March 1, 1945, and before March 1, 1985. Wyoming’s Excess Water Law states that each water right with a priority date of post-March 1, 1945, but pre-March 1, 1985, is entitled to 2 cfs per 70 acres before any water is made available to post-March 1, 1985, water rights.

If there is not sufficient water to furnish 2 cfs to each post-March 1, 1945, and pre-March 1, 1985, water right, but there is more than enough to furnish 1 cfs to each of these rights, the excess water is divided among the rights on a pro-rata basis. If there is so little water that each post-March 1, 1945, and pre-March 1, 1985, water right cannot receive 1 cfs, the rights are regulated on a strict priority basis. Post-March 1, 1985, water rights are entitled to 1 cfs per 70 acres only after all pre-March 1, 1985, rights have received2 cfs per 70 acres.

When additional streamflow is available, it is not illegal to divert more than one’s amount of appropriation when prior rights are satisfied, the stream is not in regulation, and such water is not wasted as determined by the division superintendent or the local water commissioner.


Angus M. Thuermer Jr. is the natural resources reporter for WyoFile. He is a veteran Wyoming reporter and editor with more than 35 years experience in Wyoming. Contact him at or (307)...

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  1. Would this then result in two separate New Fork Lakes, as the morraine that currently separates upper and lower New Fork Lake isn’t very deep in late summer. This proposal scares me. If it benefits irrigators, then they should shoulder the financial burden, not the general public.