The St. Francis Dam was a concrete gravity dam located in San Francisquito Canyon in Los Angeles County, California, United States, built from 1924 to 1926 to serve Los Angeles's growing water needs. It catastrophically failed in 1928 due to a defective soil foundation and design flaws, triggering a flood that claimed the lives of at least 431 people. The collapse of the dam is considered to be one of the worst American civil engineering disasters of the 20th century and remains the second-greatest loss of life in California's history, after the 1906 San Francisco earthquake and fire.