Hohokam canals flow outward from the Salt River like the tentacles of a giant octopus. They split and split again, once full of gently flowing water transported for mile after mile by the forces of gravity. Omar Turney, a city engineer for Phoenix, mapped the ancient Hohokam irrigation systems during the 1920s. He walked over the river basin and consulted old maps and historical records to reveal what he called "the largest single body of land irrigated in prehistoric times in North or South America." The lower Salt River Valley, where downtown Phoenix now stands, supported miles of irrigated fields and dozens of farming communities. The scale of the irrigation works boggles the imagination. In the downtown metropolitan area alone, 300 miles of canals formed 14 irrigation networks that watered 256,000 acres of fertile river basin soils. The Gila River Valley to the south, with its four irrigation networks, watered nearly 19,000 acres of closely packed fields a thousand years ago. In the heart of this carefully engineered landscape, stood the 250-acre Snaketown site, with its ceremonial ball court. Six miles of irrigated land and smaller settlements lay along the river upstream and downstream of Snaketown. The dense cultivation extended as much as two miles from the riverbanks.These canal systems developed over a period of 600 years, from 500 to 1100 AD, so even though it took an immense amount of work to dig them it could have been done by village communities working on their own behalf. Once the systems were built, though, maintaining them must have taken quite a bit of central planning. Archaeologists once imagined that this work was done at the command of mighty priest kings, possibly from Mexico -- the Toltec-style ceremonial ball courts at Snaketown reinforced this idea. But there are no palaces or grand temples at any of these sites, nor any tradition of kingship among the Indians of this area, so perhaps this was all organized by committees of chief and elders in just the way it was done among the surviving Pueblos in the 17th century.
The Hohokam system began to break down in the 12th century. Probably because of severe droughts, some areas were abandoned and the larger settlements shrank. Around 1450 AD the whole system was abandoned. The general view is that the falling population created a downward spiral, in which the reduced population could no longer maintain the irrigation system, which decayed further, leading to smaller crops, causing more people to leave, etc. , until the system collapsed.
Which brings us to modern Phoenix. One of the disturbing things we have learned from the climate scientists is that the US southwest is prone to severe, widespread droughts that last as much as 40 years. Right now all of the water that falls in the Colorado River basin is drawn off for human use even in years when the rains are plentiful, and we still have to supplement our supply with fossil water drilled from deep aquifers. What happens when the next 40-year-drought hits?