As the nation's thirst continued to grow, even more was required of water managers—and nowhere more so than in California. The land of the gold rush and sunny skies, of rich alluvial soils and seemingly limitless opportunities, had one major problem—it didn't have nearly enough water. The case was the worst in Los Angeles, where a steadily increasing population and years of uneven rainfall were straining the existing supply from the Los Angeles River. To deal with the problem, the city formed its first official water department in 1902 and put just the right man in the job of superintendent and chief engineer. William Mulholland had moved to Los Angeles in the 1870s as a young man and had worked as a ditch tender on one of the city's main supply channels. In his new capacity he turned first to improving the existing water supply, adding reservoirs, enlarging the entire distribution network, and instituting the use of meters to discourage the wasting of water.

But Mulholland's vision soon reached further, and in 1905 citizens approved a $1.5 billion bond issue that brought his revolutionary plan into being. Work soon began on an aqueduct that would bring the city clear, clean water from the Owens River in the eastern Sierra Nevada, more than 230 miles to the north. Under Mulholland's direction, some 5,000 workers toiled on the project, which was deemed one of the most difficult engineering challenges yet undertaken in America. When it was completed, within the original schedule and budget, commentators marveled at how Mulholland had managed to build the thing so that the water flowed all the way by the power of gravity alone. At a lavish dedication ceremony on November 5, 1913, water finally began to flow. Letting his actions speak for him, Mulholland made one of the shortest speeches on record: "There it is. Take it!"

Los Angeles took what Mulholland had provided, but still the thirst grew. Indeed, throughout the 20th century communities in the American West took dramatic steps to get themselves more water. Most notable is undoubtedly the combined building of the Hoover Dam and the Colorado River Aqueduct in the 1930s and early 1940s. The dam was the essence of multipurposefulness. It created a vast reservoir that could help protect against drought, it allowed for better management of the Colorado River's flow and controlled dangerous flooding, and it provided a great new source of hydroelectric power. The aqueduct brought the bountiful supply of the Colorado nearly 250 miles over and through deserts and mountains to more than 130 communities in Southern California, including the burgeoning metropolis of Los Angeles. Other major aqueduct projects in the state included the California Aqueduct, supplying the rich agricultural lands of the Sacramento and San Joaquin valleys. The unparalleled growth of the entire region quite simply would have been impossible without such efforts.