San Francisco has maintained water rights to the Tuolumne River, a pristine source that originates high in the Sierra Nevada, since the early 1900s. Now, those rights are being threatened by a state proposal to rescue the Sacramento-San Joaquin River Delta. The state’s plan, which was first proposed in October 2016 and released as a final draft last month, would increase water flows to the Tuolumne and other rivers that support the delta.