MODESTO - Originating in a glacier at the eastern edge of Yosemite, the Tuolumne River runs into a man-made roadblock in the towering granite cliffs of the Hetch Hetchy Valley. A massive concrete dam captures its icy water and ships much of it through pipes and tunnels to the residents of San Francisco. Further downstream, the Tuolumne is halted again, this time by a dam in the oak-covered Sierra foothills. From there, a network of canals spreads the Tuolumne’s waters over mile after mile of rich San Joaquin Valley vineyards, orchards and dairy farms. The meager amounts left in the...