SAN FRANCISCO -- At the end of her tenure as mayor of this fermenting metropolis, Dianne Feinstein probably thought she had seen all the stimulation life could hurl her way. Then the telephone on her desk rang, and from across the continent in Washington the secretary of interior said he had a neat idea for turning off the city's water and electricity.

Well, not really. Without prejudging his idea -- which he stresses is only that, an idea, not a proposal -- this should be said: Secretary Donald Hodel is having Second Term Fun. His idea for dismantling a dam and draining Hetch Hetchy reservoir is the sort of thing that can only be thought in a president's second term, when fatigue has everyone feeling a bit flaky and they don't mind raising some dust.

The Hetch Hetchy water covers what once was a dazzling valley especially loved by John Muir, founder of the San Francisco-based Sierra Club. The valley would be the worse for wear after six decades under water, but Hodel rightly thinks it would be fascinating to watch nature restore it, as the slopes of Mount St. Helens are being restored.

Feinstein tartly calls Hodel's ''the worst idea since the sale of weapons to Iran.'' Hodel's Cabinet colleague, Secretary of Energy John Harrington, clearly thinks the idea is crackers because it would cost billions -- perhaps $6 billion -- to find alternative water supplies and new sources of electricity, the sale of which is important to San Francisco.

The Auburn dam to the north would have to be completed. But it has been stalled for 12 years because of worries about its ability to withstand earthquakes and because of the opposition of environmentalists who also hate the dam at Hetch Hetchy. Completing Auburn might cost $1 billion; dismantling what there is of it might cost half that. The dam at Hetch Hetchy is crucial to a marvelous engineering achievement which, with 155 miles of tunnels and pipelines, delivers water to the nation's fourth largest metropolitan region. Hetch Hetchy water is renowned for its purity (everyone says it tastes great and is less filling) and is, says Feinstein, a San Franciscan's ''birthright.''

Hodel is the environmentalists' 3 a.m. nightmare, a conservative with political and personal skills James Watt lacked. But with his Hetch Hetchy idea, he has flanked environmentalists on the left. Such is the suspicion, not to say paranoia, of some environmentalists that there are dark rumors about Hodel's wanting to drill for oil on the valley floor. More reasonably, some people see his idea as an irresistible (to a conservative) way of discomfiting a city that is soggy with political and cultural liberalism. You want environmentalism? Better buy some candles and bottled water.

Actually, Hodel comes on as James Herriot, friend of chipmunks and all other creatures great and small, including the Americanus Touristus, a herd-traveling, camera-slung biped that flocks to Yosemite valley in inconvenient numbers, causing the valley to be as congested as a shopping mall.

Hodel rightly says that adding a million acres to Yosemite would not diminish congestion in the valley because everyone wants to see El Capitan and the other famous splendors. Solution: Add a valley. Feinstein argues that adding a valley with a floor a mile-and-a-quarter long will not appreciably alleviate congestion in the more famous valley. And anyway, it if did, it would make Hetch Hetchy valley into something that Muir would not have wanted.

Hodel has started a splendid row, not least among environmentalists. However, Congress must be the dam-buster and, even were it to approve, the project would take 10 years, by which time the Big One (the anticipated earthquake, that is) may have rearranged San Francisco. So there is no immediate need to panic, at least about Hetch Hetchy.

Feinstein is almost certainly right on the merits, but merits aren't everything, and Hodel's idea has a lovely disproportion, expressing an almost heroic indifference to mere practicality. The idea that the country in its current fiscal condition should spend $6 billion to dismantle an engineering marvel in order to dry out a pretty little valley is, coming from a conservative, piquant.

However, in recent decades many environmentalists have spread for themselves a big blanket on the moral high ground and had a picnic, suggesting that anyone who talks in terms of trade-offs (urban needs vs. conservation; tourist desires vs. preservation) is insensitive. Hodel has rained on the picnic, driving a lot of people indoors to talk about trade-offs.