An escalating legal fight over a $59 million federal dam project on Montana's lower Yellowstone River could decide the fate of an endangered, dinosaur-like fish population that has been blocked from its spawning grounds for decades.

Construction on the dam northeast of Glendive is set to begin in September.

Environmentalists this week asked U.S. District Judge Brian Morris to block the work. They say a proposed bypass channel meant to let pallid sturgeon circumvent the dam could fail, dooming the population of aging fish that can reach five feet in length.

The U.S. Army Corps of Engineers and Interior Department say the two-mile channel would allow sturgeon to access an additional 165 river miles upstream for migration and spawning. Government biologists say that is a potential lifeline for the recovery of a population that has dwindled to an estimated 125 wild fish. There's also an unknown number of younger, hatchery-raised sturgeon.

On Friday, irrigators who serve farmers in eastern Montana and western North Dakota sought to intervene in the case on the side of the government. Water diverted from an existing barrier known as a rock weir at the dam site serves 57,000 acres of cropland in the two states, according to an affidavit filed by attorneys for the Lower Yellowstone Irrigation Project.

The 125 sturgeon that inhabit the lower Yellowstone have been essentially trapped downstream of the rock structure since it was built in 1905. At least one female fish managed to swim around the structure during high water last year, but that was considered a rare occurrence.

"We think if the dam is built and it doesn't work, it's game over for the sturgeon," said Steve Forrest with Defenders of Wildlife, which joined the Natural Resources Defense Council in a lawsuit filed in February against the federal agencies involved in the project.

The two groups on Wednesday requested an injunction prohibiting construction while Morris considers their claims that alternatives to a dam were not considered.

Attorneys for the government and environmentalist in May reached an agreement to try to resolve the groups' request before Sept. 8, after which the Corps intends to issue a notice for construction to begin.

The government proposal includes construction of a solid concrete weir that would stretch across the river and the two-mile-long bypass. The environmentalists contend that alternatives — such as using pumps to deliver irrigation water into a nearby canal — should have been given more consideration.

Lower Yellowstone irrigators' attorney Mark Stermitz said the bypass channel offered the best choice, both for helping sturgeon and protecting the future of the farms served by the water project.

Pallid sturgeon was listed as an endangered species in 1990. Believed to date to the days when Tyrannosaurus Rex walked the Earth, the species has declined sharply during the past century as dams were built along the Missouri River system.