Still, water districts in Utah, New Mexico and Colorado propose five major pipes to drain another 691,000 acre-feet a year.
"You want to think that over very carefully before you invest a billion dollars," said NRDC senior policy analyst Barry Nelson, co-author of the report with University of New Mexico law professor Denise Fort.
"They have to pay for those projects," Nelson said of the ratepayers and taxpayers seeking the water. "They're stuck with the bill, but the reliability of those projects is very questionable."
Each acre-foot of water in the arid West is generally considered enough to supply a household for a year.
This new wave of big water proposals is different in at least two respects from the previous one that tamed the river with the likes of Glen Canyon and Hoover dams, according to the report. First, the pipelines don't piggyback on major new storage reservoirs, meaning they don't add to water availability. Second, they rely on energy to pump water, instead of tapping hydropower dams to cut costs.
Those more cost-effective mega-projects have already been built, Nelson said, and now water conservation is the better bet.
"The water world is different today than it was 150 years ago," he said.
But developers argue conservation won't solve their problems.
Ron Thompson, general manager of the Washington County Water Conservancy District, told the Utah Legislature's Public Utilities and Technology Interim Committee on Wednesday that the St. George area has exhausted its options for growth.
"If we reach a half-million [Washington County residents], as the state projects by 2040 or 2050," Thompson testified, "clearly we're going to have to have another water source."
Lake Powell, fed by the Colorado, seems the most dependable place to look, he said. Treating the county's salty groundwater with reverse osmosis would cost more.
Conservationists and smart-growth advocates counter that Utah's Dixie can step up conservation at the same time that it inevitably converts irrigated farmland to homes, eliminating the pipeline's need. And the NRDC report criticizes a state study of the proposal for largely ignoring conservation despite a regionally high per-capita water use of 430 gallons a day.
Thompson said water-use comparisons to other Southwestern cities are unfair because St. George sends a higher percentage of its water to institutions. Residential use averages 110 gallons, he said.
Conservation won't sustain even St. George's "organic growth," Thompson said, let alone a return to pre-recession levels of migrating retirees.
To read the Natural Resources Defense Council's report, go to http://tinyurl.com/7s4ddlo