Below is a satellite view of the previous location of the St. Francis dam, a ways north of Los Angeles, CA. (Click image to enlarge)
The city of Los Angeles had built an aqueduct to deliver water from the Owens Valley, which was a couple hundred miles north of LA. There was need for a reservoir at the end of the aqueduct, for local storage of water, rather than constantly having to adjust the flow to meet demand.
To meet this need, the St. Francis dam was built in the San Francisquito Canyon. Unfortunately, the sides of the dam were built into prehistoric landslides. These landslides were unstable and permeable to water, The dam began leaking as soon as the reservoir was filled, and continued to do so for two years, until it finally failed.
The video below gives a brief description of the nature of the failure, and the Wikipedia article on it is here.
St. Francis Dam before failure.
The dam's rapid failure sent a huge wall (over 120ft high) of water rushing downstream. The water flooded the towns of Saugus, Newhall, Fillmore, and Santa Paula. The damage was enormous, and the death tool was high. All the result of an engineering screw-up, and compounded by hubris.
Quote from the Wikipedia article (I highly recommend reading the entire thing!)
To this day, the exact number of victims remains unknown. The official death toll in August 1928 was 385, but the remains of victims continued to be discovered every few years until the mid-1950s. Many victims were swept out to sea when the flood reached the Pacific Ocean and were never recovered, while others were washed ashore, some as far south as the Mexican border. The remains of a victim were found deep underground near Newhall in 1992, and other bodies, believed to be victims of the disaster, were found in the late 1970s and 1994. The current death toll is estimated to be a maximum of 431 victims.