What Happened to the Ancient Sonoran Desert People?

Today we visited the Casa Grande ("Big House") Ruins National Monument in Coolidge, AZ. It was the first prehistoric and cultural site to be protected by the US Government in 1892. The Big House pictured above (on the left in 1880 and on the right today protected by a steel roof), marked the peak of the Ancient Sonoran Desert People's civilization. Archeological dating methods estimate that it was constructed in the 1300's. Shortly after its construction, the Sonoran Desert civilization collapsed. What happened?

During the late 1300’s and early 1400’s, the ancient Sonoran Desert people suffered a period of widespread depopulation and abandonment. Speculations as to the cause have included drought, floods, disease, invasion, earthquakes, internal strife, and salinization of farmland. Today, several American Indian groups have ancestral links to the ancient people. Their cultural traditions, together with on-going archeology and the continued interest of visitors at Casa Grande Ruins, all combine to keep the legacy of the ancient Sonoran Desert people alive to this day (from the NPS here).

There is no definitive scientific answer but a few things struck us as we visited the site:

• The ancient Sonoran Desert people constructed (using sticks, stones and human labor only) an impressive irrigation system around the Gilla River (graphic above). Digging irrigation ditches in the Sonoran Desert was no easy task since the layer of caliche below the surface soil usually takes picks, shovels and jack hammers to break up today--tools not available to the ancient Sonoran Desert people. What would have motivated a population to undertake this brutal work?
• The Big House is at least a mile from the Gilla River path today which has no year-round running water as a result of the Coolidge Dam.
• The timbers to support the four-story Big House were carried by humans from surrounding mountainous areas many miles away where trees from which 12 foot lumber could be cut were available. Hundreds of heavy timbers had to be moved hundreds of miles by hand to build the floors of the four-story building. Only a repressive mini-Empire would have been able to extract that kind of brutal labor from a desperate population.
• Prior to construction of the Big House, people lived in small, single story dugout shelters. The Big House would not have been used for human shelter but rather for the extraction and storage of surplus.

What struck us after taking all this in was that the Big House we were gawking at was the arrogant proclamation of an ancient mini-Empire that could command such an impractical and useless monument on the basis of overpopulation supported by irrigation technology. Just at the peak of their civilization, the ancient Sonoran Desert people had created a impressive but unstable civilization that was easily toppled. Whether it was toppled by drought, flood, disease, invasion, earthquakes, internal strife or salinization of irrigated farm land doesn't matter as much as how easily and quickly the civilization collapsed.

The remaining families returned to an earlier, more sustainable civilization based on tribes and small family units that became the current Native American Tribes of the Southwest. I wonder if the politicians in the State Capital (Phoenix, AZ), the current pinnacle of Southwestern US Civilization, are aware of the lessons to be learned from studying the ancient civilizations in their backyard? My guess is that they are convinced that civilizations will never collapse again because our overpopulation is supported by advanced technology.