REASSEMBLING HETCH HETCHY: WATER SUPPLY WITHOUT O'SHAUGHNESSY DAM 1

The Hetch Hetchy System provides San Francisco with most of its water supply. O'Shaughnessy Dam is one com- ponent of this system, providing approximately 25 percent of water storage for the Hetch Hetchy System and none of its con- veyance. Removing O'Shaughnessy Dam has gained interest for restoring Hetch Hetchy Valley. The water supply feasibility of removing O'Shaughnessy Dam is analyzed by examining alternative water storage and delivery operations for San Fran- cisco using an economic engineering optimization model. This model ignores institutional and political constraints and has perfect hydrologic foresight to explore water supply possibili- ties through reoperation of other existing reservoirs. The eco- nomic benefits of O'Shaughnessy Dam and its alternatives are measured in terms of the quantity of water supplied to San Francisco and agricultural water users, water treatment costs, and hydropower generation. Results suggest there could be lit- tle water scarcity if O'Shaughnessy Dam were to be removed, although removal would be costly due to additional water treat- ment costs and lost hydropower generation. (KEY TERMS: water supply; dam removal; Hetch Hetchy; opti- mization; economics; filtration avoidance; restoration.)