The federal government is considering endangered species status for four snails and a limpet believed to live only along a 40-mile stretch of the Snake River in southern Idaho's Magic Valley.

The U.S. Fish and Wildlife Service was expected to make a decision soon on the petitions for endangered status that have been filed by private interests over the past 10 years.The service's staff has recommended protection because of threats to the mollusks in the Thousand Springs area of the river, primarily because of proposed developments that include two power plants and a trout farm.

Protection would not only halt those developments but could dramatically affect other traditional uses of the river, and a spokesman for Republican Sen. James McClure maintained that if the species are threatened by development then those developments should be addressed before granting protected status to the species and creating potentially severe economic disruption.

The petitions for protected status were discovered by The Post Register in Idaho Falls.

The staff recommendation proposes protection for the Bliss Rapids snail, the Utah valvata snail, the Snake River physa snail, the Idaho spring snail and the Banbury Springs limpet, a mollusk with a cone-shaped shell and a thick, fleshy foot.

All five are only found in the cool, clear, free-flowing waters of a 40-mile stretch in the Thousand Springs area or large adjacent springs, biologists say.

They are the remnants of what was once a diverse population of 90 mollusk species that lived in Lake Idaho during the Pleistocene period some 12,000 years ago. The Banbury Springs limpet, also known as the lanyx, is believed to be the only representative of its genus left in the state. Others of that genus have been found only as fossils from Lake Idaho.

"It's very exciting," said Peter Bowler, a California biologist who grew up in the Hagerman area. "That genus has not been seen in this area since the Pleistocene."

But just as important as the species themselves is the ecosystem in which they reside, a clean, free-flowing stretch of river that supports among other species white sturgeon, says one of the federal biologists who has recommended protecting the snails.

"The snails are representative of a group of animals in the Hagerman reach that require top-quality habitat, flowing water with high levels of oxygen," said Jay Gore, Fish and Wildlife endangered species director for Idaho. "They are a good indicator of the health of the ecosystem."

Threatening that habitat are two hydroelectric projects - the A.J. Wiley Project proposed above Bliss by Tacoma Power and Light Co. and the city of Idaho Falls and the smaller Dike project below Bliss - and a private fish hatchery in Box Canyon, said Vicki Finn, a Fish and Wildlife biologist in Washington, D.C.

"The dams are the biggest threat because they could wipe out the habitat," Finn said.

Gore said some of the snails were present downstream but disappeared following construction of the C.J. Strike Dam.

Fish and Wildlife officials briefed members of McClure's staff earlier this month, and the senator's natural resource specialist Carl Haywood said McClure's involvement was routine and not an attempt to put pressure on the agency against listing.

"If they actually are endangered they should be protected," Haywood said. "If they're not, then they shouldn't."

Gore said agricultural runoff, other irrigation diversion projects and general habitat destruction also threaten the mollusks' survival.