From above, tracing the Colorado River along the Arizona-California line in an airplane, it’s easy to see how it happened. As the river bends and weaves through the Southwest, its contents are slowly drained away. Concrete canals send water to millions of people in Phoenix and Tucson, Los Angeles and San Diego. Farms, ribbons of green contrasted against the desert’s shades of brown, line the waterway. Farther downstream, near Yuma, the river splits into threads, like a frayed piece of yarn. A massive multistate plumbing system sends river water to irrigate the hundreds of thousands of farm acres in Southern California and Arizona, hubs for winter vegetables, alfalfa, cotton and cattle.