The cold, rushing water of the Tuolumne River, piped from the high peaks of Yosemite to the taps of Bay Area residents, is not only among the nation’s most pristine municipal water sources but extraordinarily plentiful. This point of pride for San Francisco, which has maintained rights to the cherished Sierra supply since the early 1900s, is being threatened, however. Under a far-reaching state plan to bump up flows in California’s rivers, the city would be forced to limit its draws from the Tuolumne for the first time in recent memory.