CHEYENNE, Wyo. — The U.S. Bureau of Reclamation's initial assessment of how much water could be diverted in coming decades from the Green River basin in Wyoming is far less than the amount a private entrepreneur has applied to draw for a planned pipeline to Colorado, agency officials said.

Developer Aaron Million, of Fort Collins, Colo., has proposed building a private pipeline to carry up to 250,000 acre feet of water a year from the Green River. He proposes to move 225,000 acre feet to Colorado's Front Range and 25,000 acre feet to eastern Wyoming.

An acre foot is about 325,000 gallons. Million's $3 billion proposal has drawn fierce opposition in southwestern Wyoming. Sweetwater County and the cities of Green River and Rock Springs are pooling hundreds of thousands of dollars to fight the project, citing concerns about its effect on fishing and recreation in the area.

The U.S. Army Corps of Engineers is working on an environmental impact study on the likely effects of building the pipeline. Officials say the study should determine exactly how much water is available.

The Bureau of Reclamation, which manages Flaming Gorge Reservoir, conducted a preliminary study in early 2007 in response to Million's proposal to draw water from the reservoir and the river upstream. It pegged the basin's annual water availability at 85,000 acre feet less than what Million proposes to draw.

The study found that up to 165,000 acre feet of water could be drawn from the reservoir each year through 2049 while still allowing enough water to generate hydropower and to meet the needs of endangered fish species downstream.

Drawing 165,000 acre feet a year over 40 years would draw the level of Flaming Gorge Reservoir down from its current elevation of 6,030 feet a bove sea level to just above 5,900 feet, the study found. According to bureau figures, the reservoir has a surface area of nearly 30,000 acres when its elevation is at 6,000 feet and its surface area would drop to just under 11,000 acres at the 5,900 foot level.

After 2050, as development in Wyoming increases, the bureau study projected Flaming Gorge Reservoir could sustain an annual diversion rate of 120,000 acre feet. The agency is updating that draft analysis, and agency officials say their projections of the amount of available water may well go lower.

Malcolm Wilson, chief of the Water Resources Group for the Bureau of Reclamation in Salt Lake City, said the bureau's draft study doesn't support Million's plans.

"I think Mr. Million's aware of our numbers, and we're not sure where he gets his," Wilson said. "Certainly what we saw in this study, as you can see, does not support his numbers. But maybe he has something else."