The last time the Hetch Hetchy Valley emerged from 300 feet of Sierra water was during the severe drought of 1991.

To quench the Bay Area's thirst, San Francisco water officials sucked the reservoir almost dry. For a brief time they uncovered the glacial valley that had inspired paintings and prose a century before.

But in 1991, the Hetch Hetchy Valley looked more dead than alive.

One-hundred-year-old tree stumps studded the barren landscape. A dusting of silt and pebbles covered the valley floor. There were no signs of the valley's lush meadow. Gone were the groves of oaks and pine. The valley that naturalist John Muir championed in the early 20th century was unrecognizable.

Congressmen didn't listen to Muir in 1913, when he lobbied to leave Hetch Hetchy Valley intact for the American public as part of Yosemite National Park. They allowed San Francisco to build a dam and flood it in 1923. Only on unusual occasions, when serious droughts demand it, does the valley emerge again from its underwater fate.

Hetch Hetchy, the smaller twin of Yosemite Valley, might look dead on those occasions, but it's not, according to federal biologists who studied the matter. Its state is rather like that of a deep sleep.

A team of scientists from U.S. Fish and Wildlife Service, the California Department of Fish and Game and the National Park Service came together in 1988 to study the matter. Their job was to examine a controversial proposal by Donald Hodel, President Reagan's secretary of the interior. Hodel wanted Hetch Hetchy restored for the national park.

San Francisco leaders howled in protest. Hodel got nowhere with his idea. But credit him and the scientists who prepared the Interior Department report. They figured out the science of the restoration if not the politics.

A restoration of Hetch Hetchy wouldn't be a quick makeover. The scientists examined the main issues and concluded:

• The dam must stay, or at least a very large section of it must remain. San Francisco dug 118 feet below the riverbed to build the foundation for the dam. "The removal of the lower 118 feet of the dam would vastly change the river gradient at the narrow lower end of the valley and would probably lead to rapid erosion of the meadows in the lower chamber of Hetch Hetchy," the scientists said.
• The sediment isn't as big a problem as one might think. On many rivers, a dam will capture tons of loose dirt and small rocks and transport the sediment toward the sea. That didn't happen at Hetch Hetchy, which is a good thing. If it had, the valley would be more dead than alive. The sediment load "appears quite low," the scientists said. "The Tuolumne River descends from a watershed comprised largely of thin soils and great expanses of exposed and glaciated rock." (In 1991, barely an inch of sediment covered the floor.)
• The river channel probably remains. "The aquatic ecosystem of the Tuolumne River will return to near pristine conditions without management intervention," the scientists said.
• Two options exist for grasses, plants and trees. Let nature do the job, or manage what grows back. By leaving things alone, "within two years extensive areas on the floor of Hetch Hetchy valley would be covered with grasses, sedges and rushes. ... Willows would begin to colonize the riverbanks." The drawbacks: Grasses wouldn't be native grasses, and the native pines and oaks might face some competition. If the valley were managed, after five years, "conifers would be up to 15 feet high and black oaks would be about six feet high in areas planted the first year."
• The valley would have a "bathtub ring," but it wouldn't last forever. Eighty-one years of storing water has left a line along the granite walls. "It is the result of impounded water killing the native rock lichen colonies, which cover the granite walls. Natural restoration of such colonies would take between 80 and 120 years."
• Wildlife would return, possibly at breakneck speed. Deer would return in the first year and black bears soon afterward.

As the scientists reported, awakening Hetch Hetchy is not a physical impossibility. It is a political challenge, and one that is receiving a fresh look by the University of California, Environmental Defense and others. They are unearthing some surprisingly achievable options, such as relying on three other dams on the Tuolumne River to store the water Hetch Hetchy supplies for the Bay Area today. Legislators have shown an interest: This week the head of the California Assembly's water committee, Joseph Canciamilla of Pittsburg, and Assemblywoman Lois Wolk of Davis, both Democrats, asked Gov. Arnold Schwarzenegger to study Hetch Hetchy anew.

The fate of a spectacular valley in a national park is worth another look. Restoration would certainly take years, even decades. But as a natural marvel, united once again with the Yosemite Valley to the south, Hetch Hetchy would be something to behold.

Muir said it best in 1890: "Imagine yourself in Hetch Hetchy. It is a sunny day in June, the pines sway dreamily, and you are shoulder-deep in grass and flowers. Looking across the valley through beautiful open groves you see a bare granite wall 1,800 feet high rising abruptly out of the green and yellow vegetation and glowing with sunshine, and in front of it the fall, waving like a downy scarf, silver bright, burning with white sun-fire in every fiber. ... It is a flood of singing air, water, and sunlight woven into cloth that spirits might wear."

For now that scene is a memory, a national treasure hidden away, underwater. It doesn't have to be that way. With political champions, the vista could become a reality once more, a place to be experienced and savored by all who visit our national park.