1 - The St. Francis Dam was a concrete gravity-arch dam, designed to create a reservoir as a storage point of the Los Angeles Aqueduct. It was located in San Francisquito Canyon of the Sierra Pelona Mountains, 40 miles (64 km) northwest of Los Angeles, California, near the present city of Santa Clarita. 

The dam was built between 1924 and 1926 by the Los Angeles Department of Water and Power, then called the Bureau of Water Works and Supply, under the supervision of the department's chief engineer and general manager William Mulholland. Two and one-half minutes before midnight on March 12, 1928, the dam failed catastrophically, and the resulting flood killed more than 450 people. The collapse of the St. Francis Dam is one of the worst American civil engineering failures of the 20th century and remains the second-greatest loss of life in California's history, after the 1906 San Francisco Earthquake and fire. The disaster marked the end of Mulholland's career.

2 - Prelude to disaster
 
Throughout 1926 and 1927, several temperature and contraction cracks appeared in the dam as the reservoir filled. The cracks and leaks were inspected by Mulholland and his assistant, Harvey van Norman, and judged to be within expectation for a concrete dam the size of the St. Francis.
 
Through the closing months of 1927 to March 1928 the reservoir rose steadily and uneventfully. On March 7, 1928 the reservoir had reached full capacity; Mulholland ordered that no more water be turned into the St. Francis. 
 
Dam keeper Tony Harnischfeger discovered a new leak on the morning of March 12. Concerned that the muddy color of the runoff he observed could indicate the water was eroding the foundation of the dam, he immediately alerted Mulholland. After inspecting the leak Mulholland and Van Norman determined that the muddy appearance of the water was not from the leak itself, but came from where the water contacted loose soil from a newly cut access road. Convinced that the leak was not a danger, Mulholland and Van Norman pronounced the dam safe.

3 - Collapse and flood wave 

Two and one-half minutes before midnight on March 12, 1928, the St. Francis Dam catastrophically failed.
 
Although there were no eyewitnesses to the dam's collapse itself, a motorcyclist named Ace Hopewell had ridden past the dam, by his approximation ten to fifteen minutes prior. In his testimony at the Coroner's Inquest, he stated he rode up the canyon and had passed both power house 2 and the dam without seeing anything which would cause concern. Approximately one mile (1609m) upstream of the dam he heard, above the sound of his motorcycle, what to him was much like "rocks rolling down the mountain." He stopped and checked the hillsides above him. The sound which he had heard earlier was behind him and he assumed this had possibly been a landslide as these were common to the area. It is believed that he was the last person to have seen the St. Francis Dam intact.
 
4 - The dam keeper and his family were the first casualties caught in the flood wave, also called dam break wave, which was at least 125 ft (38 m) high when it hit their cottage in San Francisquito Canyon, approximately 1/4 mile (400 m) downstream from the dam. Thirty minutes before the collapse a motorist passing by the dam also reported seeing lights in the canyon below the dam —the dam itself did not have lights—suggesting Harnischfeger may have been inspecting the dam immediately prior to its failure. The body of Leona Johnson, who lived with the Harnischfegers (and has been often and mistakenly reported to be Harnischfeger's wife) was found fully clothed and wedged between two blocks of concrete near the broken base of the dam. Neither Harnischfeger's body nor that of his six-year-old son, Coder, were ever found.

Source English: En.wikipedia.org