Two California Dams: A Comparison of M. M. O’Shaughnessy’s Hetch Hetchy Dam and William Mulholland’s St. Francis Dam

In the 1920s San Francisco and Los Angeles reigned as California’s premier cities. During the 1920s both built major concrete curved gravity dams, massive structures that helped symbolize the growing importance of their respective urban centers. But the fate of these two dams was quite different: San Francisco’s O’Shaughnessy Dam remains today a vital component of the Bay Area’s municipal water supply system, while Los Angeles’ St. Francis Dam catastrophically failed on March 12, 1928, killing some 400 people. This paper illustrates how the two dams differed in their design—especially in regard to uplift—and also notes how an exemption included in California’s 1917 dam safety, allowed engineers M. M. O’Shaughnessy (for San Francisco) and William Mulholland (for Los Angeles) the freedom to build them without any review or approval by the State Engineer or other state authorities.