illiam Mulholland began his career in 1878 as a zanjero, or ditch tender, for the City of Los Angeles, one of an army of men tasked with minding the network of troughs that brought the Los Angeles River to the city’s taps. He was brilliant, teaching himself mathematics, hydraulics, and geology in his off-hours. By 1923, he was head of an ambitious project that aimed to construct the world’s largest concrete-arch dam. When it was finished in 1926, the St. Francis Dam held some 12 billion gallons of water a mere 10 miles north of what is now Santa Clarita. It promised to solve LA’s water woes for the next century, sustaining the population through any drought. Instead, just before midnight on March 12, 1928, the dam broke loose, unleashing a 100-foot wall of water that tore its way through the towns and countryside between it and the coast. Thousands of people lay sleeping in its path.