William Mulholland was the engineer who built the Los Angeles Aqueduct, the 233-mile channel that brought Owens Valley water to the city beginning in 1913. The aqueduct made Los Angeles what it became. In the years that followed, Mulholland designed and built storage infrastructure throughout the basin to hold and distribute that water. The Hollywood Reservoir was one of those projects. Its dam — a curved concrete gravity dam 210 feet high and approximately 933 feet along the crest — was completed in 1924 and named in Mulholland's honor.

The site is in the Santa Monica Mountains at the head of Weidlake Drive, above what would become the Hollywoodland development and eventually just Hollywood. The reservoir itself, now called the Hollywood Reservoir or Lake Hollywood, covers about 177 acres and holds approximately 2.7 billion gallons. The surrounding land was designated the Lake Hollywood Park, and the reservoir became a hiking destination — a quiet body of water inside one of the most urbanized regions in the world.

On March 12, 1928, at approximately 11:57 p.m., the St. Francis Dam gave way. Mulholland had inspected it that very morning and declared it sound. The failure sent a wall of water forty feet high into the Santa Clara River valley. It moved at eighteen miles per hour. It reached the Pacific Ocean near Ventura five and a half hours later. The official death toll was placed at 431; some estimates run higher. The disaster remains one of the deadliest in California history.

The cause was geological: the eastern abutment of the dam was anchored in a formation of ancient landslide debris that behaved like a lubricant when saturated. Mulholland had not recognized it. He testified at the coroner's inquest and accepted full responsibility, a statement of personal accountability that also functioned as a professional epitaph. He resigned shortly afterward and spent the remaining years of his life in effective retirement. He died in 1935.

In the immediate aftermath, the city moved to inspect all its other dams, including the one in the Hollywood Hills. In 1933 and 1934, as a precaution, engineers added 330,000 cubic yards of earth fill to the downstream face of the Mulholland Dam — reinforcing a structure that had not failed, using what had been learned from one that had.

The Hollywood Reservoir is as close to a secret as a body of water can be within a major city. It sits behind a fence above the commercial density of Hollywood, surrounded by hills that belong to the Santa Monica Mountains Conservancy. The reservoir path, roughly three miles around the water, became one of the more popular urban walks in Los Angeles — accessible enough to be crowded on weekends, unusual enough to feel like an escape.

The dam and reservoir appeared in Roman Polanski's Chinatown (1974), the film that used the water wars of early Los Angeles as the frame for a story about corruption and power. The reservoir also appeared in Earthquake (1974) and Valley Girl (1983). The dam was added to the National Register of Historic Places in 1974, fifty years after its construction. Mulholland Drive, the road that runs across the crest of the Santa Monica Mountains above it, was named for the same engineer whose legacy is inseparable from the question of what happens when a city grows faster than its infrastructure, and when the man who built that infrastructure trusts his own judgment one time too many.