An armada of giant yellow earthmovers on the prairie south of Denver is racing to dig one of Colorado’s biggest water-supply reservoirs in decades — a hole 180 feet deep across 1,400 acres — designed to wean suburbs off waning aquifers.

But the water to fill this reservoir?

Not yet secured.

The prospect of what critics call an empty bathtub is generating anxiety around Colorado as water managers clash over the last unclaimed mountain river flows.

Most water to fill the Rueter-Hess reservoir “will have to be imported,” said Frank Jaeger, manager of the Parker Water and Sanitation District, who for 25 years has led the effort to supply 450,000 suburban residents.

Importing water would require multibillion-dollar pumping and piping from rivers running down the western side of the Continental Divide, such as the Colorado, back across mountains to Front Range residents, Jaeger said.

Though huge, the costs likely would be less than for alternatives such as trapping and treating contaminated water from the South Platte or Arkansas rivers, he said.

The option Jaeger and a Colorado-Wyoming coalition of municipal suppliers favor — one of four being considered by state natural resources officials — would divert water from the Flaming Gorge Reservoir in western Wyoming along Interstate 80 to Colorado.

Groundwater dropping

If water can’t be imported to the reservoir, an estimated 190,000 Front Range residents who currently rely on groundwater face an uncertain future, as water tables are dropping 30 feet a year.

Douglas County communities “need to wean themselves off groundwater because it is finite. You don’t want to have your lifestyle depend on a resource that is finite,” said Bob Raynolds, a Denver Museum of Nature & Science researcher who for years has monitored aquifers. “We know we’re falling down the slope of diminishing returns.”

Suburban municipal wells, drilled over the past 20 years as deep as 2,745 feet through bedrock, today produce far less than they did a few years ago.

Yet Colorado Western Slope leaders see the $230 million Rueter-Hess reservoir as folly — and bristle at talk of diverting more water across the mountains to fill it.

The reservoir “is 20 times more expensive, and 10 times as big as they need. It’s going to be a little bit of water in a big bathtub,” said Eric Kuhn, manager of the Colorado River Water Conservation District, based in Glenwood Springs.

The financing, based on tap fees from anticipated housing construction, “is the water equivalent of a Ponzi scheme,” Kuhn said.

State demographers project Colorado’s fastest growth will be on the Western Slope, with populations in the Colorado and Yampa river basins nearly tripling by 2050. Water demands to meet urban and energy industry needs there are expected to double.

A “fairy tale”

“There’s a very good chance that, in the long run, there’s not going to be any more water available on the Western Slope. And, if they’re having trouble now paying for Rueter-Hess, how are they going to pay for moving water from the Western Slope? That’s why I say this is a fairy tale,” Kuhn said.

The push to build the reservoir began in 1985 when Parker obtained a required federal permit for construction in Newlin Gulch. Then other south metro suburbs signed up. The size was quadrupled, with federal approval.

It was presented as storage for recycled water and Cherry Creek storm runoff during wet years — water that normally would be lost downriver.

Now the emerging hole is sufficient to store 72,000 acre-feet. An acre-foot is equal to 325,851 gallons of water, or enough to supply two families of four for a year. Rueter-Hess will be ready for water by 2012, said Doug Voss, project superintendent for Weaver General Construction Co.

Revolt over cost

South metro residents ultimately will have to pay for the project and for water, Jaeger said.

“There’s no cheap way out of this situation,” Jaeger said.

That’s touchy. Parker-area residents revolted this year. A proposed 28 percent hike to cover operating costs prompted a recall campaign to oust water board members. An election is set for Dec. 15.

“All the providers recognize the cost of water is going to be significantly more,” said Rod Kuharich, director of the 13-member South Metro Water Supply Authority.

This month, more construction vehicles are rolling into action to build up the 7,700-foot-wide Frank Jaeger Dam at the reservoir.

Critics “can make their claims,” but the reservoir will be crucial to sustain population growth, Jaeger said.

Paying off the debt for the construction now underway all depends on tax revenues from future growth, he said.

“To say, ‘We’ll just shut off growth’ will only exacerbate problems,” he said. “If you don’t pay off debt, what do you do? What does that do to the economy of the whole state? We need steady, controlled growth. All our needs for a reasonable lifestyle are tied into this.”

Bruce Finley: 303-954-1700 or bfinley@denverpost.com