Cornelia F. Hugel Professor of History Lafayette College
Hoover Dam (built 1931-35) stands as a premier example of massive curved gravity concrete dam technology, but its design was predicated on a long period of development dating to the 1870s and continuing through the 1920s. This paper analyzes two of the most important concrete gravity dams built in California in the 1920s: the St. Francis Dam built to store water for the Los Angeles Aqueduct and the Hetch Hetchy Dam (later renamed “O’Shaughnessy Dam”) built to store water for San Francisco’s municipal water supply. These dams were built by two of America’s most famous hydraulic engineers of the 20th century—William Mulholland of Los Angeles and M.M. O’Shaughnessy of San Francisco—and they experienced very different fates. The St. Francis Dam collapsed horrifically in March of 1928, unleashing over 12 billion gallons of water into the Santa Clara Valley north of Las Angeles and killing some 400 people; the Hetch Hetchy Dam remains in service today, 100 years after its initial construction, providing high quality water to residents of San Francisco and the nearby Bay Area. This paper examines and compares these two famous California dams, specifically considering: 1) their technological features, 2) contemporary understanding of how “uplift” forces act on gravity dams, and 3) their relationship to California’s dam safety bureaucracy of the 1920s. The paper’s goal is to provide important contextual analysis of gravity dam technology in the era immediately preceding construction of Hoover Dam. It also highlights the enormous responsibility that dam engineers assume when building large scale water storage structures.
