Unidentified tourists picnic in San Francisquito Canyon following the collapse of the St. Francis Dam on March 12-13, 1928.

A steady stream of looky-loos flocked to the dam site from the time the roads reopened until the remaining section of the dam, aka the "tombstone," was dynamited in May 1929 — and afterward. More than a few folks thought it was a suitable place for a picnic.

We don't know many particulars about this photograph. It's part of an anonymous grouping from a garage sale, which included this one, showing tourists visiting the tombstone.

Judging from the contents of an envelope included with the nearly 600 photographs in this garage-sale lot, the image can be sourced to a Russell Ryan of 3560 Opal Street in the Boyle Heights section of Los Angeles. According to the 1940 Census, Ryan was a woodworker in a wood working shop, born about 1902 in Canada. His wife, Charity, was born about 1896 in Illinois, and they had a son, Russell Jr., born about 1925. The elder Ryan had his photographs developed at Sunrise Pharmacy, 1162 S. Lorena St. in Boyle Heights, Thomas H. Peffer, proprietor.

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.