The cities and towns hit this week with stiff demands to reduce water use, including San Francisco, say they’ll work with state regulators to meet the charge, but they’re also looking at the possibility of lawsuits.

The State Water Resources Control Board approved a far-reaching plan Wednesday to improve the health of California’s rivers and fish by limiting the amount of water that dozens of communities take from four major waterways.

While the plan leaves room for negotiating the extent of the water reductions, the agencies that draw from the San Joaquin River and its tributaries say legal action may be a necessary backstop in case they’re forced to cut more than they can afford.

“At the end of the day, we do serve our customers and we have to do what’s best for the community,” said Samantha Wookey, spokeswoman for the Modesto Irrigation District, one of the state’s biggest water suppliers and now subject to restrictions on the Tuolumne River.

San Francisco, which also relies on the Tuolumne River and faces cutbacks, has begun evaluating whether a lawsuit is appropriate, according to the city attorney’s office.

Under the state plan, San Francisco residents and businesses could face reductions of 40 percent or more during prolonged dry periods, according to estimates from the San Francisco Public Utilities Commission.

The aim of the state water board is to prevent the collapse of the Sacramento-San Joaquin River Delta. The sprawling estuary, which serves as a hub of state water supplies and is a vital conduit for threatened salmon, has suffered from too little water and too much contamination amid heavy pumping.

While the water board’s plan would saddle water users with restrictions to boost flows into the delta, regulators introduced leniency Wednesday in an effort to head off litigation, which would result in years of delay in the plan’s implementation. The board stipulated that it will attempt to integrate proposals by water agencies to trade habitat restoration for smaller water reductions.

It appears, though, the board’s strategy may not have worked.

Ever since the water board began updating its plan a decade ago, it has had to maneuver a path between water users who don’t want limits on their draws and fishermen and environmentalists who want substantial caps.

The conservation community also presents a threat for legal action, with many having committed to suing if the environmental safeguards approved Wednesday are weakened.

The Bay Delta Plan calls for maintaining an average of 40 percent of the natural flow of the San Joaquin River and its tributaries during peak spring runoff. Currently, the flows average 20 percent or less because of diversions. Sometimes the waterways dry up entirely.

The plan could change as the state looks at amendments pitched by water agencies.

For years, the state board has been urging suppliers to come up with their own ideas for fixing California’s declining river system. At Wednesday’s meeting, more than a dozen water departments, with help from the state Natural Resources Agency, laid out a framework for restoration that included money for habitat improvements and a commitment to less pumping. The board asked the agencies to flesh out their initiative by March.

Officials at the SFPUC, who are working on amendments, say they won’t be able to hit the state’s 40 percent flow target. But they say they’re looking to leave more water in the Tuolumne River and restore salmon habitat in a plan they believe will meet the water board’s conservation goal.

“There are other areas we can explore on how we do long-term management,” said Michael Carlin, deputy general manager of the SFPUC. “That’s where we’re going to be spending our time over the next several months: to get something more detailed.”

Still, the compromise plan probably would mean significant water cuts for San Francisco households. During a drought, customers may have to reduce water use by 30 percent, Carlin said.

Water rates would probably rise as the city develops new sources to make up for lost supplies.

The agency serves about two dozen other Bay Area cities that would face the likelihood of similar reductions and rate hikes.

The state water board is expected to begin determining the exact restrictions for water agencies once the suppliers submit their alternative ideas. Next, the board will have to enshrine those limits in regulation. The water rights of many suppliers may have to be adjusted, which will require additional proceedings.

Much of the expected legal action will probably revolve around these water rights.

The Trump administration, which is pushing to free up more water for farms, has also threatened to sue over the Bay Delta Plan. On Thursday, Bureau of Reclamation Commissioner Brenda Burman said her agency is analyzing what impact the state’s action might have on federally managed water.

In addition to the San Joaquin River basin, the state has begun a similar process of limiting draws on the Sacramento River and its tributaries.

Peter Drekmeier, policy director for the Tuolumne River Trust and an advocate of higher water flows, said he’s pleased with what the state has come up with, at least so far.

“We see this as a big step forward, but it’s a long process and we’re not going to rest on our laurels,” he said. “The lawsuits are going to be a big thing in the next couple months.”

There’s still a lot that could happen, he said.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander