Construction of San Luis Dam lasted from 1963 to early 1968. The dam was constructed in several zones with quarried local earth and rock averaging as much as per day. The 100-ton Caterpillar dump trucks used during construction were, at the time, the largest in the US. The construction of the four water intake towers, each high, involved the first use of tower cranes in the US. The work force peaked at 2,304 in October 1965 and declined thereafter; in January 1968, with construction almost completed, the first water was delivered through the California Aqueduct to begin filling the giant reservoir. The dam was officially dedicated on April 20, 1968 by Secretary of the Interior Stewart L. Udall. However, it was not until May 31, 1969 when San Luis Reservoir filled for the first time.