Water users in San Francisco and its suburbs face a day of reckoning as state regulators move to leave more water in California’s two biggest rivers in an effort to halt a collapse in the native ecosystem of the San Francisco Bay and its estuary, the Sacramento-San Joaquin River Delta. Even as water allocations to California farmers have been severely reduced, San Francisco water authorities have freely tapped the Tuolumne River, which the city dammed early in the last century at its headwaters in Yosemite National Park.