SHASTA LAKE, Calif.—Few places are more critical to the water supply in California than this immense northern reservoir in the foothills of the Cascade Range. Fed by runoff from 14,163-foot Mount Shasta and other peaks, California’s largest reservoir opened in 1945 as part of the federal Central Valley Project, an elaborate system of man-made dams, pumps and aqueducts that aims to reduce flood risks and deliver water to farms and cities in the heart of the semiarid state.

---