The rivers that once poured from the Sierra Nevada, thick with snowmelt and salmon, now languish amid relentless pumping, sometimes shriveling to a trickle and sparking a crisis for fish, wildlife and the people who rely on a healthy California delta. A state plan to improve these flows and avert disaster, however, has been mired in conflict and delays. And critical opposition is coming from an unexpected place: progressive San Francisco. City water officials worry that the far-reaching effort to revive hundreds of miles of waterways will mean giving up too much of their precious mountain supplies.