"Dam before the break" by George Watson. Undated photo. Early 4x5-inch copy negative from the Watson Archive.

Note the cars and pop-up tent on top of the dam.

Dam historian Frank Rock (2017): "From the looks of the fresh debris below the dam on the left, my guess is this is a picture of the opening dedication of the dam in 1926."

Dam historian J. David Rogers (2017): "I think this is 1926 because the lake level is a good 50 feet below spillway, and I think the vehicles may be associated with the construction of the steel safety railings across the dam crest."

Construction on the 600-foot-long, 185-foot-high St. Francis Dam started in August 1924. With a 12.5-billion-gallon capacity, the reservoir began to fill with water on March 1, 1926. It was completed two months later.

At 11:57:30 p.m. on March 12, 1928, the dam failed, sending a 180-foot-high wall of water crashing down San Francisquito Canyon. An estimated 411 people lay dead by the time the floodwaters reached the Pacific Ocean south of Ventura 5½ hours later.

It was the second-worst disaster in California history, after the great San Francisco earthquake and fire of 1906, in terms of lives lost — and America's worst civil engineering failure of the 20th Century.