SAN FRANCISCO (AP) - Of all the battles waged over natural resources in California, perhaps none is bolder, or more romantic, than a campaign by environmentalists to tear down a dam in Yosemite National Park that has provided water and electricity to much of Northern California for 80 years.

A report released Monday by Environmental Defense is the latest attempt to sway public opinion in favor of draining Hetch Hetchy Valley and restoring to nature what conservationist John Muir called Yosemite Valley's little brother - a "precious mountain temple" and "grand landscape garden" that now lies 300 feet under water.

The authors of the report, "Paradise Regained," argue that water quality, supply and storage, as well as power generation, could be maintained if the Hetch Hetchy Valley, in the Sierra Nevada mountains about 160 miles east of San Francisco, were drained and restored. The study proposes a variety of alternative water and power sources for the San Francisco Bay area and the Central Valley if the O'Shaughnessy Dam were taken down.

The report comes as the Bay Area embarks on a $3.6 billion expansion and retrofit of the Hetch Hetchy water system, which seismologists warn could leave millions without drinking water if a major earthquake strikes.

"We have an amazing opportunity to return Yosemite's second crown jewel to the American people," said Tom Graff, Environmental Defense's California regional director. "It's not technically or institutionally simple, but we think it can happen if federal, state and regional agencies cooperate."

The report was met with a mixture of curiosity, skepticism and downright opposition from officials in the Bay Area and the Central Valley, which rely on the Tuolumne River and the Hetch Hetchy reservoir for drinking water, irrigation and hydropower. They pointed out the daunting technical, financial and political obstacles to revamping the system.

"The concept of tearing down Hetch Hetchy infrastructure and restoring that valley might excite the imagination, but it needs a reality check," said Kate Hora, a spokeswoman for the Modesto Irrigation District, which supplies water from the Tuolumne River to farmers and residents in the Modesto area.

Sen. Dianne Feinstein, D-Calif., said she was opposed to "the destruction of one of the largest sources of clean drinking water in California.

"In a state that has faced repeated droughts and is desperate for water sources, I believe this would be a terrible mistake," she said in a statement.

The debate over Hetch Hetchy Reservoir has stirred passions since 1913, when Congress passed the Raker Act, which authorized the damming of the Tuolumne River despite fierce opposition from conservationists.

The Hetch Hetchy system provides some of the nation's highest quality drinking water to more than 2.4 million residents in San Francisco and its fast-growing suburbs in San Mateo, Santa Clara and Alameda counties.

The first serious look at restoring Hetch Hetchy came in the mid-1980's when Interior Secretary Donald Hodel commissioned a study that concluded the valley would return to life within a decade if the dam were taken down. But the idea was shelved after it encountered intense opposition from powerful water interests.

Over the past year, the campaign to restore Hetch Hetchy has gained momentum with a University of California, Davis study published on alternative water sources, and a recent series of editorials in the Sacramento Bee advocating the valley's restoration.

Environmental Defense decided to conduct its own study after San Francisco officials declined to do a joint study two years ago.

The report, written by researchers at Environmental Defense and three consulting firms, concludes that the Bay Area could receive a majority of its drinking water and hydropower by using other reservoirs on the Tuolumne River. In especially dry years, water supplies could be obtained by boosting Bay Area storage capacity and water purchases from agricultural districts, as well as storing water in underground reservoirs.

The San Francisco Public Utilities Commission, which manages the Hetch Hetchy water and power system, released a statement saying it was sympathetic with the study's goals, but urged caution.

"The report today paints a very optimistic and rosy picture of the minimal impact on the Bay Area," SFPUC spokesman Tony Winnicker said. "The reality is much more complicated. At what cost does it all happen? And who's going to pay?"

Environmental Defense says its report is only meant as a starting point for discussion. The group hopes that federal, state and local agencies will agree to a more comprehensive study that could put a price tag on the project.

"Imagine the opportunity to allow nature to recreate another place like Yosemite Valley. Why not at least take a look?" said Ron Good, executive director of Restore Hetch Hetchy. "It was done by humans. It can be undone by humans."

On the Net:

Environmental Defense: http://www.discoverhetchhetchy.org

San Francisco Public Utilities Commission: http://www.sfwater.org