At first blush, the idea appeared so radical that it might have sprung from the pen of John Muir himself: Tear down the dams that put Yosemite's Hetch Hetchy Valley under water more than a half-century ago and restore the spectacular valley that the famed naturalist spent the last six years of his life trying to defend.

But the suggestion came out of a personal computer operated by Interior Secretary Donald Hodel, and it took conservationists so much by surprise that many of them were unable to decide yesterday whether to take the notion seriously or condemn it as a political act of desperation.

In a recent memo to departmental officials, Hodel said the draining of the Hetch Hetchy Reservoir, which provides water to San Francisco and more than 30 other small communities, would be a "thrilling project" and a "tremendous payoff for America."

Hodel said that restoration of Hetch Hetchy would create another Yosemite Valley, a scenic gorge so popular with park visitors that weekend sightseers often must endure hours-long traffic jams below its soaring granite cliffs.

"What a fantastic contribution that would be," he wrote. "What an incredible, irreplaceable addition to the national park system."

Environmentalists interviewed yesterday fervently agreed. But the memo also suggested a tradeoff that raised as many suspicions as the Hetch Hetchy idea raised expectations.

To replace the water and power provided by Hetch Hetchy, Hodel offered the possibility that San Francisco could be persuaded to help pay for the nearby Auburn Dam, a long-dormant Bureau of Reclamation project that has become an expensive albatross at the Interior Department.

Auburn Dam was left half-completed in the 1970s after conservationists raised questions about its environmental impact and ability to withstand earthquakes.

Completing it could cost from $535 million to $1.4 billion, Interior Department officials said, perhaps more if the dam had to provide water and power to San Francisco as well as its proposed use as a flood-control project for Sacramento.

But tearing down the dam could cost as much as $500 million, and leaving it in place is costing the department $1 million a year in maintenance.

"I guess my initial impression is that it's a desperate attempt to keep the Bureau of Reclamation in business. I can't imagine that it would be deemed acceptable," said Ed Osann of the National Wildlife Federation.

"There are assumptions on this laced all through the secretary's musings, and one is that the Auburn Dam is essential for flood control in Sacramento," Osann said. "There is no evidence yet that Auburn is the only way or even the best way to provide that."

Paul Pritchard, president of the National Parks and Conservation Association, was equally skeptical, accusing Hodel of offering to rectify a past mistake while failing to protect natural resources from threats under the Reagan administration.

"If we realize that this is a mistake, which it was, why are we sacrificing the other Hetch Hetchys?" he said, citing Hodel's promotion of oil drilling in the Arctic National Wildlife Refuge. "I don't think anybody should attribute any virtuous motive to this. It's a political gambit."

The question was a little knottier for the Sierra Club, which sprang into national prominence in the early 1900s when Muir, its founder, took on the Forest Service in a protracted -- and ultimately unsuccessful -- effort to save the Hetch Hetchy Valley from inundation.

A club spokesman said Hodel did not mention Auburn Dam when he laid out the idea in a telephone call to board chairman J. Michael McCloskey. "He did some story telling, and then he said, 'What do you think of it?' " said spokesman Joann Hurley. "Well, what were we supposed to say?"

"The Auburn Dam is a lousy idea," said board member Denny Shaffer, who added that the group was nonetheless interested in exploring the concept of dismantling the Hetch Hetchy dams. "Whatever this is, it isn't a plan," he said. "If folks are serious, then we need to go to the next step."

While the conservation groups were stepping out their careful minuet, however, some politicians had their positions well staked out.

San Francisco Mayor Dianne Feinstein told The New York Times, which disclosed Hodel's plan yesterday, that it was "the worst idea I have heard since the sale of weapons to Iran."

Rep. George Miller (D-Calif.), chairman of the Interior subcommittee on water and power resources, said the memo was "so ludicrous it simply doesn't merit any comment."

But Rep. Richard H. Lehman (D-Calif.), whose district includes Yosemite National Park, called it a "visionary idea" and said he believed Hodel was sincere in proposing it.

"This is something environmentalists have wanted since the dam went up: to tear it down," he said. "He {Hodel} puts people in a difficult position. How do you oppose him just exploring it, if there's a chance you could mitigate the awful damage at Hetch Hetchy? You look unreasonable if you say you can't.

"Hodel has made it something that can be talked about in the future," Lehman said, and on that point the environmentalists concurred.

"The notion of decommissioning some of these facilities is a good one in concept," said the wildlife federation's Osann. "Perhaps the secretary's memo will stimulate some thinking about that."