A multi-million dollar effort to divert water from a Colorado River tributary into southwest New Mexico is raising concerns, with some reports putting the cost at $1 billion and critics saying it offers far fewer benefits than expected.

Supporters point out that a 1964 Supreme Court ruling entitles New Mexico to the water, which ultimately flows into neighboring Arizona, and that the water is needed for agriculture, drinking and other essential needs. 

Critics argue it would be far too costly, that diverting the water could cause environmental problems and that the river is rarely high enough to divert water. 

They largely cite state and federal reports including one in July 2014 by the Interior Department that found “estimated costs exceed estimated benefits” in the dozens of submitted proposals.

“New Mexico should not use any more federal funds on this … over-budget, ineffective diversion project,” the watchdog group Project on Government Oversight, or POGO, concluded after reviewing the reports.

The state informed the Interior Department in November 2014, just weeks before the deadline to get federal money for the project, that it was going forward with the effort.

The agency’s Bureau of Reclamation says such an effort, which would include one or more dams, will cost $762 million to $775 million.

The Interstate Stream Commission, a lead state agency on the project, has not returned several requests for information, specifically cost estimates.

State lawmakers also appear concerned about escalating costs and sticking taxpayers with the bill, despite supporters of the project insisting they will find private revenue sources.

“It won’t be just $1 billion,” state Democratic Rep. Bill McCamley said at a Sept. 1 meeting on the issue.

Darr Shannon, a member of the Central Arizona Project New Mexico Unit, the quasi-government group helping lead the diversion effort, said at the meeting that she and others would find the private money.

She told FoxNews.com on Wednesday that the diversion effort is needed, in part, to make sure residents have a future water supply.

Critics also point to two reports requested by the governor-appointed Stream Commission that also identified potential problems.

The reports, by technical experts, concluded that each of the roughly 45 submitted proposals would be hundreds of millions of dollars over budget and take decades to complete -- with no guarantee of water for residents in the region of vast desert basins and mountain ranges.

“The cost of the dams could be underestimated by more than 100 percent,” water resources engineering firm RJH Consultants told the commission. “It is our opinion that the overall project cost may be 25 to 50 percent higher than the current estimate.”

In addition, the potential benefits would amount to $74 million to $458 million, according to POGO.

The 649-mile-long river flows from southwest New Mexico into Arizona, before emptying into the Colorado River.

The 51-year-old Supreme Court ruling allows New Mexico to divert some Gila River water.

But the money didn’t come until decades later, through the 2004 Arizona Water Settlements Act. The measure was approved by Congress with the stipulations that New Mexico gets the full amount only if it builds a diversion project and applied for the money before December 2014.

Another concern is that the Settlements Act states New Mexico can divert Gila River water only if Arizona users have sufficient water.

Expert testimony in some of reports suggests that the Gila has flowed high enough to meet diversion requirements just nine percent of the time since 1936 and that New Mexico won’t see the benefits of the diversion project for nearly 20 years.