I lead my group closer to the deceptively tranquil water, still hoping that the sun will burn through the cold fog. I turn to my main subject: the force of the water and the scour of the tides. Through the mist, we can see a stretch of mudflats and marsh.

"It hasn't always looked like this," I say, and pass around a black-and-white photograph taken 70 years ago on the very spot where we stand. The change is startling. Where willets and marbled godwits now thrust their bills into the mud searching for invertebrates, the picture shows cattle grazing on a broad expanse of pastureland.

"In the early '80s, this area became part of the Reserve. The levees were breached, and the land returned to a more natural state." I point to a low railroad bridge that allows the only tidal flow to this 460-acre portion of the slough. After years of study, it is here that a first attempt will be made to turn back the tide.

Bryan Largay has been working to make this happen. Largay directs the Tidal Wetland Project, which seeks to conserve and restore estuarine habitats at the slough. He described to me plans to build a giant, horseshoe-shaped underwater sill of steel pilings at the railroad bridge. Largay's team hopes that the $2 million construction project, which is funded by federal stimulus money and scheduled to begin in the fall, will decrease the volume of water entering this part of the slough. He says that slowing the tidal flow here should also help to alleviate some of the scouring elsewhere.

Despite sophisticated modeling projections, predicting changes in the slough's complicated hydrology is like one of those math problems with so many parts that it makes your head hurt. The underwater sill is the first cautious step in trying to slow the erosive tides. What is learned from this project will help guide future restoration plans.

By the end of the afternoon, I'm once again standing with the tour group at the overlook. The sun has finally dissolved the fog and filled the air with warmth. As the slough's stillness seeps into us, the group finds an easy camaraderie. No one is in a hurry to leave.

They gather around me, and I tell them a final story: "Those of us who work at the slough sometimes jokingly call it the ‘cosmic center of the universe.' " I smile, then gesture toward the mosaic of deep-green oaks and amber grasslands rolling down to a nearby marsh where a great flock of sandpipers wheel and flash in the sun. "I guess the cosmic center could be anywhere you want it to be. But if you had to pick a place..."

After the group wanders back to the visitors' center, I linger at the overlook. A pale crescent moon softens the sky, and the tide follows the silent lunar traction. Another mud bank crumbles, another truckload of marsh slips into the sea.

Despite all the human alterations to the slough, I know there's an even darker, shadowy history of what might have been. During the 1960s and '70s, bitter battles were fought to stop developers from lining the slough's shores with condominiums and boat docks and from constructing a major oil refinery and a nuclear power plant.

I've come to believe that preserving and restoring the slough's former private pastureland as a public nature preserve represents more than a physical transformation. It embodies the radical idea promoted a century ago by John Muir, Theodore Roosevelt and Gifford Pinchot, Roosevelt's chief forester, to set aside great tracts of land as reserves and refuges for the public and for the wildlife that live there. It's what New York Times environmental writer Timothy Egan describes as "the West of possibility versus the West of possession."

I look across the braided channels of water and marvel that it's been seven years since I first stood here. The tides rise, the tides fall, the years pass. When I am an old man, I want to be able to return to this overlook, to bring my grandchildren here to breathe the air fragrant with salt and sage and to have them hear the curlew's call.

John Moir is an award-winning author and journalist who lives in Santa Cruz, California; see www.jmoir.com