REDMESA – Construction of the dam designed to corral 5,100 acre feet of runoff from two modest streams in this arid section of La Plata County is expected to be completed in July – two years after groundbreaking.

Long Hollow Reservoir will be a water bank against which irrigators in the area can draw. They will be able to pull more water from the La Plata River, which must be shared with New Mexico because the reservoir can make up the difference.

No potable water will be available. Residents have wells for household water or they buy from water docks.

Brice Lee, president of the sponsoring La Plata Water Conservancy District, said the district has been pursuing the Long Hollow project since the 1990s when the irrigation-water component was removed from the larger and seemingly interminable Animas-La Plata Project, known as A-LP.

“It’s not the best solution, but it’s better than nothing,” Lee, who has raised cattle and grown hay in the Hesperus area for 57 years, said of Long Hollow.

Potentially, 500 to 600 irrigators could be interested in reservoir water, he said. A fixed fee would be set to cover maintenance and operations, plus a charge based on consumption.

Irrigators who don’t go for the backup source of water will continue to take their chances with the fickle La Plata River.

The reservoir will store water from Long Hollow Creek and Government Draw, which drain 43 square miles east of Colorado Highway 140. The reservoir is about five miles north of the New Mexico line and a half-mile from the confluence of Long Hollow Creek and the La Plata River.

An outlet on the left side of the dam feeds the natural channel of Long Hollow Creek below the dam, a U.S. Fish & Wildlife Service requirement aimed at maintaining aquatic life.

Water also can be diverted into a high-flow pipeline if water demands from New Mexico exceed 10 to 12 cubic feet per second or if an emergency release were required.

It was first estimated that the project would cost $22.5 million. The pot consisted of $15 million set aside by the Colorado Water Resources and Power Development Authority for future projects when the A-LP was downsized. Accrued interest and $3 million from the Ute Mountain Ute Tribe completed the budget.

But a bill making its way through the state Legislature is expected to contribute an additional $1.575 million to cover the expense of meeting unexpected difficulty in readying the dam’s bedrock foundation for construction.

The dam is 151 feet high with a span of 800 feet. A central clay core is buttressed upstream and downstream by tons of sand, dirt and rock.

Construction, which began in July 2012 with excavation down to bedrock, was followed by filling with grout under pressure fissures in the bottom and embankments of the dam to prevent leaking. Some grout holes were bored as deep as 120 feet.

All construction material, with the exception of steel and concrete, come from on-site sources.

The capricious flow of the La Plata River has produced verbal shoving matches between Colorado and New Mexico since the signing in 1922 of the compact that requires the states to share the river.

Each state has unrestricted use of the water from Dec. 1 to Feb. 15. But from then until Dec. 1, if the river is flowing at less than 100 cubic feet per second at the state line, Colorado must deliver one-half the flow at Hesperus to New Mexico.

Living up to the terms of the agreement isn’t easy.

The La Plata River, which tumbles from its origin high in the mountains north of U.S. Highway 160, isn’t the most generous of sources at best.

A porous river bed and thick vegetation grab an inordinate share of the flow.

The growing season is longer than the period of river flow.

An experiment – routing the La Plata River through Cherry Creek for a stretch and then back into the La Plata channel – hasn’t left everyone happy. Long Hollow Reservoir could prove a more satisfying solution.

The dam was designed by GEI Consultants, a national firm with a branch in Denver. The Weeminuche Construction Authority is the builder.

Among the 50 crew members, 80 percent are Native American, with 65 percent being Ute Mountain Utes, said Aaron Chubbuck, the Weeminuche project manager.

The construction engineer, hired by the water district, is Rick Ehat, who brought the A-LP to completion on time and on budget after an earlier administration fell disastrously behind on both counts.

The finished dam may appear a monolithic structure. But it’s actually an amalgamation of “zones” comprised of dirt, rock, sand and clay with each ingredient serving a certain purpose.

After the topping-out ceremony marks the completion of construction, the “borrow areas” where construction materials were taken will have to be revegetated. Also, certain electrical and mechanical work remains to be done. Among the tasks, sensors will be installed on the downstream face of the dam to measure possible movement or leakage.

“There’s no allowance for error,” Ehat said. “The reservoir will be filled slowly and the dam checked at different levels.”

Dams are built to handle three situations, Ehat said. Those situations:

The dam must support the force exerted by normal loading.

The dam must be able to handle a draw down (the release of water) when saturated earth and sand on the upstream face of the dam want to pull away from the structure.

The dam must withstand flooding.

Seismic consideration must be considered, but it’s not a controlling factor in this case, Ehat said.

Unlike the Lake Nighthorse, the A-LP reservoir, which was filled by pumping water from the Animas River, Long Hollow Reservoir will depend on precipitation runoff and return flow from agricultural operations.

The construction used 900,000 cubic yards of material, compared with 5.4 million cubic yards for Ridges Basin.

While useful for its purpose, the 5,100 acre-feet of water behind Long Hollow dam is peanuts compared to the 123,541 acre-feet in Lake Nighthorse and the 125,000 acre-feet in Vallecito Reservoir.

Depending on the weather, Ehat said, it could take five to seven years for the reservoir to fill from runoff from Long Hollow Creek and Government Draw.

daler@durangoherald.com