About 7,600 acres or 12 square miles in northern Las Vegas is subject to the review that pits environmentalists versus city officials. Development of about 6,000 acres in North Las Vegas could also be curtailed as the BLM eyes protecting fossils and rare plants in the northern valley.

The BLM has hired Utah State University to conduct a 30-month study analyzing the potential environmental impact of developing near the Upper Las Vegas Wash. The federal agency had appeared to settle on 5,300 acres for a conservation area, but to the delight of environmentalists, and dismay of cities and homebuilders, the BLM has left the door open to expand any conservation area to more than 13,000 acres.

That has Las Vegas officials concerned about future growth. If the BLM preserves most of the land in and around the Upper Las Vegas Wash, the city would have only about 300 acres in the corridor available for development, said Chris Knight, Las Vegas' administrative services director.

"We become landlocked if we are not able to develop in the 12 square miles," said Knight said. "The whole valley is absorbing 6,000 to 7,000 acres a year. If we had that, it would give us several years of growth."

The delay hurts Las Vegas more than Henderson and North Las Vegas because those cities have other areas available for development, Knight said.

Las Vegas had been preparing to spend millions of dollars on designing and installing water and sewer lines and other infrastructure to serve the area, but the BLM's study has postponed those plans for now. If the city waits for the BLM to finish its work, any federal auction of the land is at least six years away, officials said.

"The question is do we spend now not knowing where the conservation boundary is and put the money at risk," Knight said. "It is frustrating because there is a lead time in designing infrastructure. Before you have the development, you have to have the infrastructure designed and built."

The BLM conducted an environmental study of the Las Vegas Valley in 2002 when Congress expanded the amount of federal lands to be auctioned by 22,000 acres. The study showed the Upper Las Vegas Wash had sensitive resources that needed protection, prompting the BLM to identify more than 5,000 acres to be removed from federal auctions until further study determined the conservation area's exact boundaries.

Many suspected the BLM would choose a conservation area of 5,300 acres, but the agency surprised observers earlier this year when it called for a lengthy review that put off the decision on the boundary for at least two more years.

"I know it is frustrating for some, but we have a mission to manage the sensitive resources for future generations," said Gayle Marrs-Smith, a BLM botanist and manager of the project.

The Las Vegas Valley was once a shallow marsh, but with much of it now developed, the Upper Las Vegas Wash is the one place left where fossils can be preserved, Marrs-Smith said. Some fossils of tiny sea creatures are up to 200,000 years old, she said.

"This is absolutely a treasure trove of past information," Marrs-Smith said. "If we can preserve intact and study them, we can understand what this area looked like thousands of years ago. It is a looking glass into the past. We can understand what the environment was like and plant community was like."

Knight said Las Vegas officials support preserving the wash but question the need to preserve more than 13,000 acres as backed by the Sierra Club and other environmentalists.

The fear in allowing too much development is that it will create runoff that damages fossils, Marrs-Smith said. Also, allowing development brings more people to the area, which might also damage the preserve, she said.

Science and public input will help decide the issue.

"We will be utilizing science, but science won't spit out an answer," Marrs-Smith said. "We will look at the alternatives and see what the consequences are. We will use that information and make a decision."

Some conservationists have suggested the area be declared a National Conservation Area similar to Red Rock Canyon west of Las Vegas or Sloan Canyon south of Las Vegas.

"I say there is a better way to grow," said Jane Feldman, conservation chairwoman of the local Sierra Club chapter. "None of us want to live in a place that only includes streets and asphalt. We have lots of places for development. We have so many unused parcels in all sorts of different places. Cities everywhere have populations growing and making use of redevelopment opportunities."

Anytime land is removed from housing development, it could push up prices, observers said. Homebuilders have proposed that the BLM preserve 5,300 acres.