As a result of an agreement between the state and agribusiness landowner Cargill Inc., miles of diked ponds and earthen levees in the southern tip of the bay will gradually be transformed to tidal marsh.

The major restoration of 16,500 acres has been likened in scope to programs in the Florida Everglades and Chesapeake Bay on the Eastern Seaboard and will be a windfall for wildlife, bird watchers, bicyclers, hikers and kayakers who will have access to a big new swath of South Bay shoreline.

Standing on the edge of a large salt pond near the town of Alviso, biologist Janet Hanson, director of the San Francisco Bay Bird Observatory, said she foresees a grand debut for this virtually undiscovered corner of the estuary.

"People could be out here riding their bikes and watching birds," she said. "It's not a very well-known place right now."

Last month, after five years of secret negotiations and plans, the state signed a final agreement to pay $100 million for land and salt-making rights from Cargill, which bought the salt ponds from Leslie Salt in the early 1970s.

The California Coastal Conservancy estimates the restoration will cost hundreds of million of dollars culled from federal, state and private sources over the course of decades.

The conservancy is holding public meetings to inform Bay Area residents about the restoration, including a session from 9:30 a.m. to 12:30 p.m. Wednesday in room M-4 of Palo Alto's Cubberly Community Center, 4000 Middlefield Road.

The meeting also presents an opportunity to suggest ways to open to both birds and visitors dozens of ponds shored up by about 200 miles of earthen levees along the shoreline of San Mateo, Santa Clara and Alameda counties.

MEETING OF MINDS

San Francisco and Point Reyes bird observatories, the U.S. Fish and Wildlife Service and Geological Survey and state Fish and Game are providing research on enhancing the bay. The Coastal Conservancy is responsible for writing a restoration plan in five years.

Right now, hundreds of thousands of shorebirds are arriving in the South Bay. The Forester's and Caspian terns pick the islands in the salt ponds for breeding, and the western sandpipers also spend time on the mudflats in front of the levees.

While the mallard, ruddy and bufflehead ducks like to forage in the shallow,

open waters of the ponds, the diving scaups and canvasbacks gather in the deeper ponds. Fish-eating California gulls, and some black skimmers and cormorants, nest on the levees.

Resident snowy plovers seek the brine shrimp and flies on the edges of the ponds. California clapper rails choose pickleweed and Pacific cordgrass for shelter along the marsh edges and sloughs.

In the reconfiguring of the ponds, scientists may turn two-thirds of the old ponds to tidal marsh. But they may also retain the remainder of the ponds.

"Despite the fact that the salt ponds are artificial, they've been around for more than 150 years. So we have entire communities of wildlife supported by them. The challenge is to retain those communities while restoring to tidal marsh," said John Takekawa, a biologist with the U.S. Geological Survey who monitors wildlife on the South Bay ponds.

A TWO-FOLD TASK

The challenge lies in trying to design an ecosystem that maintains shallow, open water habitat while at the same time converting to a tidal marsh. Some species will benefit from tidal marsh, while others will actually have decreased habitat.

"It's a balancing act. Ecological management is getting to be about trade- offs. We have less available habitat out there that hasn't been altered by development or humans," Takekawa said.

"We have a responsibility to the species lost or diminished that depended on tidal marsh. And on the other side, we can't re-create history. With 8 million people encroaching on the bay -- and 3 million in the South Bay -- the room for restoration is narrow and fragmented. What if we gain five clapper rails but we lose 10,000 shorebirds? How do we decide?" said Takekawa.

The San Francisco estuary is recognized as a Western Hemisphere Shorebird Reserve Network site for more than a million shorebirds in migration and as the winter home for more than 50 percent of some species of diving ducks in the Pacific Flyway.

With 30 species, the bay region is a waterfowl habitat area of major concern in the North America recovery plan. One of the largest populations of canvasback ducks in North America winters here.

But today's bay marsh land is just a shadow of the 190,000 acres it was before the Gold Rush. The vast habitat was once home to millions of shorebirds and ducks, American river otters, tule and Roosevelt elk and grizzly bears.

LESS THAN A QUARTER REMAINS

With California's immense growth, newcomers filled about 150,000 acres for farming, grazing and building cities. Some marshes were diked for the salt works. Flowering plants and tall grasses -- cover for wildlife -- disappeared, as did the peaty organic soils that controlled floods and filtered pollutants. Dredging opened spots for invasive nonnative weeds.

Scientists warned that the bay needed to reclaim wetlands, and in 1962 the state passed a law to control filling, giving the Bay Conservation and Development Commission authority over approvals.

Two years ago, resource agencies and environmental groups launched a campaign to add 60,000 acres of tidal marshes around the bay, bringing the total to 100,000 acres.

About five years ago, Cargill put out the word that it would sell some of its land in the south and north bays. Gov. Gray Davis and congressional leaders recognized the ecological windfall. Federal and state officials, with contributions from private foundations, secured the 16,500 acres of prime shoreline property.

Nothing has changed, yet, in the managing of the ponds. Cargill workers are still scooting around in their red trucks, shoring up the earthen levees and adjusting the levels in about 50 ponds.

REDUCING SALINITY

As part of the agreement, Cargill must bring down the salt levels to meet a new permit before the company turns the property over to the Fish and Wildlife Service. The pond water discharged into the bay must not harm the smallest bay creatures, the invertebrates.

So the Cargill workers are trying to dilute rather than concentrate the bay waters.

"It took a while to get used to the idea. You constantly had to remind yourself that you're thinking in reverse. We don't want to make salt," said Anthony "Butch" Paredes, a Cargill worker for 27 years. Paredes' father also worked there, and retired after 45 years.

"We looked at the water elevations, pumps, gates and intakes and how to manipulate the system in order to sweeten it off so it can be transferred over, " Paredes said.

Managing the ponds is more of an art than a science, he said, and he felt some qualms about dismantling a system built and designed by the people who trained him. "I'm one of two left who worked with the older guys," he said.

Now Paredes sits at the table with the scientists trying to lower the salt levels in the ponds. One of the tricks is bringing bay water into the ponds during rainy winter months, when the bay is fresher. Previously, Cargill brought in water during the summer months.

Yet Cargill has held on to 11,000 acres, including plants at Newark and Redwood City, and will maintain a production of 600,000 tons of salt a year, a figure not far below former levels. About 200 workers will remain employed.

BAY WATER INFUSION

Regulators estimate that more than half of the 16,500 acres are ready to be opened to bay waters. There are about 4,200 acres that may have elevated mercury and about 3,300 acres that may have excessively high salt concentrations.

The largest source of mercury to the bay comes from the long-closed New Almaden Quicksilver Mine. The waste flows 20 miles from the hills south of San Jose through the Guadalupe watershed and into the bay in the area of some of the Cargill salt ponds.

Yet some of the levees can't be breached. Without the levees, bay waters would flood Alviso, the little town that once canned asparagus and shipped it on barges to the rest of the country. The Bay Canning Co. remains, a relic of the past where the San Francisco Bay Bird Observatory has its office.

Biologist Hanson watched the California gulls line up on the levees, trying to find the best place to nest.

"They're not nesting, but they're thinking about it," she said. "They're saying, 'Does this look good for you, honey?' "

"It's going to be tricky," said Hanson, solving such obstacles as keeping brine shrimp and flies while opening the ponds to the fresher bay water. "But we've been monitoring the birds on the ponds for 21 years, and we know it's worth trying to keep some of these values."

E-mail Jane Kay at [email protected].