LOS ANGELES — The federal government sued California on Thursday over a water policy it said violates the state’s environmental protection law.

The U.S. Department of Justice filed suit in Sacramento federal court to block a contentious plan approved in December to increase river flows in the San Joaquin River and three tributaries to help revive dwindling salmon populations.

It was part of a larger effort to protect the Sacramento-San Joaquin River Delta, which state officials called an “ecosystem in crisis.” The delta supplies water for the majority of California’s people and farms.

Once-thriving fish in the delta, which flows out to San Francisco Bay, have plunged from some 70,000 adult Chinook salmon returning to the San Joaquin basin in the fall of 1984 to just 10,000 in 2017.

Some environmental groups supported the plan, while other conservation groups and fishing groups said it fell short in providing enough water for habitat.

Farmers opposed to the plan because it would divert less water for irrigation protested last summer outside the state Capitol.

The U.S. lawsuit said the plan was arbitrary and the state failed to analyze impacts on the environment and would reduce water coming out of the New Melones reservoir for farms, businesses and hydroelectric operations.

The Lawsuit Was an Unusual Turn of Events

The environmental analysis “hid the true impacts of their plan and could put substantial operational constraints on the Department of the Interior’s ability to effectively operate the New Melones Dam, which plays a critical role in flood control, irrigation, and power generation in the Sacramento region,” said Assistant Attorney General Jeffrey Bossert Clark said in a statement.

The federal government’s lawsuit follows a suit filed last month in state court by the California Farm Bureau Federation to block the plan.

George Kostyrko, a spokesman for the water board, said the agency looks forward to defending the plan that involved years of analysis and public input.

The lawsuit was an unusual turn of events with the state of California as a defendant in a lawsuit with the federal government. The state’s attorney general has sued the Trump administration more 47 times.

Attorney Doug Obegi of the Natural Resources Defense Council said it was ironic that the U.S. government was citing state law to sue California in federal court.

“The Trump administration has shown its flagrant disregard for state laws for some time, so it’s not surprising in that regard,” Obegi said. “I can’t for the life of me see how a federal court lawsuit against a state agency for a claimed violation of state law doesn’t get tossed out immediately for violating the U.S. Constitution.”

The NRDC said the December decision was a step forward, but it wouldn’t do enough to reach the goal to double the salmon population.

John McManus, president of the Golden Gate Salmon Association, said the plan approved in December had “re-balanced a badly out of balance water diversion scheme” and the suit was a “frivolous attempt to see more water diverted and more salmon industry jobs and families destroyed.”