The city of San Francisco is not standing down in California’s latest water war, joining a lawsuit against the state on Thursday to stop it from directing more of the Sierra Nevada’s cool, crisp flows to fish instead of people. City officials contend the State Water Resources Control Board is overreaching with a new, sweeping plan to restore California’s depleted river system by limiting draws on such water supplies as San Francisco’s Hetch Hetchy Reservoir in Yosemite. The plan, according to city estimates, will force Bay Area households to cut water use by 20 percent or more.