Courtesy Arc Ecology

On top is what Yosemite Slough might look like with a bridge. Below is what it looks like now.

The Sierra Club sued the city of San Francisco Thursday, making good on a threat to fight plans for a bridge over Yosemite Slough.

Although it's just a small part of Lennar’s plans for thousands of new homes at the old Hunters Point shipyard, the bridge became a bone of contention because it would fly over a small state park on the waterfront near the proposed development.

The Club alleges that the city’s environmental impact report, approved last month, didn’t take into account the bridge’s impacts on the surrounding wetlands and wildlife — or seriously consider an alternative route around the park. (Read the lawsuit here.)

Sierra Club member Arthur Feinstein, who had been badgering the city to take the bridge out of the project, said the purpose of the suit is to make the city reconsider its plans.

“This is not a show-stopper in any way,” Feinstein said. “This is just a little annoyance to them that they could get rid of by, for instance, getting rid of the bridge.”

The bridge is currently planned at 41 feet wide with two lanes for buses and additional lanes for bikes and pedestrians. If the 49ers decide to build a new stadium and not move to Santa Clara, the bridge could balloon to accommodate cars as well.

Courtesy Google Maps

Here is Yosemite Slough. The little blue arrow points to where the bridge would be built.

The plans raised the ire of environmental groups. But one of those groups, the California State Parks Foundation, dropped its opposition after Lennar agreed to come up with a less-intrusive design. The foundation is in the midst of restoring the Candlestick Point Recreation Area, which surrounds Yosemite Slough.

Feinstein said the Sierra Club is not so easily placated.

“We’re not saying that what they're doing may not work, but we haven't seen any pictures,” said Feinstein. “And one, there is only so much time for us to take legal action, and two, they have the approvals for this very damaging bridge.”

Matt Dorsey, a spokesman for the San Francisco City Attorney’s Office said he hadn’t seen a copy of the complaint.

Lennar has long maintained that it needs the bridge to connect its new development with local transit lines like BART and Caltrain. Local businesses have backed the idea — and Michael Cohen, the Mayor’s top economic advisor, has called it a fundamental underpinning of the project.

Meanwhile, critics like Saul Bloom, the chief executive of the environmental organization Arc Ecology, have said that buses can go around the park without any trouble — and the struggle over the bridge is really just a power play.