This forgotten part of S.F. is a long way from its best self

Thomas Avenue is an obscure street that runs through the Bayview neighborhood past Third Street, past small houses, through an industrial neighborhood to dead end at Griffith Street where there is a city facility surrounded by a high fence. It’s a neglected out of the way corner of San Francisco.

But here, just around the corner, is a small gem: Yosemite Slough, a 1,000-foot-long arm of San Francisco Bay that’s both beautiful and dangerous. It’s a backwater with a trashy past, a promising future and a curious present.

Yosemite Slough is part of the Candlestick Point State Recreation Area, but it is almost an orphan. There is barely a sign to help visitors locate the place.

The entrance and the parking lot appear closed, but the shore is open. It’s a small wonder: grassy slopes, a bike path, the blue bay beyond. In the foreground two small islets — Double Rock is the name — form the entrance to the slough channel.

To the north the old Hunters Point shipyard, to the south, Bayview hill, green and wooded. The slough itself changes with the tide. At high water Yosemite Slough is 6 feet deep, at low tide, the muddy bottom shines in the sun. It’s famously windy in the afternoon. Except for a sight of Twin Peaks on the western horizon, you would not know you’re in San Francisco.

That’s not unusual because few San Franciscans have ever heard of Yosemite Slough, which is a tidal estuary like Mission Creek, which flows past Oracle Park. The slough, gets its name from Yosemite Creek, one of those freshwater streams that lie under the city, out of sight and out of mind. Yosemite Creek, which begins at springs and a marsh in McLaren Park, is one of the smallest of these hidden creeks. It’s mostly invisible, confined to underground pipes and culverts until it meets the bay, roughly at the end of Yosemite Avenue, a deadend street in what used to be a marsh between Hunters Point and the old Candlestick Park.

In the Ohlone times, the marsh was full of wildlife. But that was long ago. Later, there was an informal park for neighborhood kids who swam and played in the bay. The Found SF website has stories about those days.

World War II hit San Francisco like a ton of bricks. The Hunters Point shipyard was taken over by the navy, the open grassy hills were covered with housing for shipyard workers, light and heavy industry filled in the flat shoreline between Third Street and the bay.

It was wartime and then a postwar boom. The military and industry didn’t worry about what damage they did to the environment. They just buried their waste or dumped it into the little slough that led to the bay: copper, lead, nickel, zinc, hydrocarbons. Not to mention raw sewage.

In the 1970s, Ruth Shaw Upton, who had moved away, came back to see her old neighborhood and looked at the pretty little Yosemite Slough she remembered. It was “a polluted cesspool,” she told Found SF, “a dismal sight.’’

In the meantime, the Bay Area became the epicenter of the environmental movement. In 1961 three women founded Save the Bay, an organization that pushed for tougher laws. Eventually the northern waterfront in San Francisco became a showpiece and even the polluted Mission Creek was lined with apartments and condos, a sort of mini-Amsterdam. Not much happened at Yosemite Slough. It was out of the way and the neighborhood, scarred by neglect, poverty and crime, was not on the civic radar.

By the turn of the 21st century, Dr. Ahimsa Porter Sumchai of the Hunters Point Community Biomonitoring Program called the slough “the dirtiest, muddiest, most hazardous area in the city and county of San Francisco.”

But things changed. The U.S. Environmental Protection Agency designated the slough as a Superfund site, and a coalition of groups including the California State Parks, the state parks foundation and private donors began restoration of the Yosemite Slough shoreline.

You should see it now. I went last week. It was kind of an exploring venture. I had never been there before and had some difficulty finding the empty parking lot at the entrance. Once inside, I walked all along the north shore of the slough, past a stately tree, past a marsh. On to the edge of the wide bay, the far shore in the misty distance.

There were only two other people there and we kept away from each other, as if by agreement. The only sound was from a junkyard at the end of the slough, a reminder of the industrial past.

Later I drove through the streets on the back end of the area to Carroll Avenue to reach the south shore, which is undeveloped, all grass and small bushes. I felt the solitude, feeling a bit wary because the neighborhood is in transition, and a city person is always aware. I wouldn’t go there at night. Work on the Yosemite Slough portion of the park stopped about two years ago until the planning and funding of a major cleanup can take place. For now this part of the park is in a state of suspended animation.

Though it has been cleaned and polished like a gem, it’s still poisoned. The water, even the mud, is still contaminated: no fishing, no swimming, no wading. I felt as if I had arrived at this corner of San Francisco a year or more too early. For Yosemite Slough the best is yet to come.

Michael Short/The Chronicle 2014

The old sewer outfall is shown at Yosemite Slough in S.F. The area has been heavily neglected through the years.

A bloom of blue-green algae floats next to buoys at Lake Temescal in Oakland. The toxic blooms were first detected in East Bay parks in 2014.