According to Leonard’s timeline, the fall probably means an eventual leveling-off of growth, and a subsequent decline. It could happen tomorrow, or centuries from now. It does not mean sudden chaos in the streets, store windows smashed out. But Leonard, in his gritty, cynical voice, does not deny the possibility.

“This is the biggest civilization here so far, the most globally connected,” he said. After another drag off his cigarette, he reminded me, “The bigger they are, the harder they fall.”

 

What could the fall look like? First, you have to look at the rise. For more than four centuries, the Hohokam were at their prime, living in a grand oasis. The Salt River flowed out of the highlands of eastern Arizona, supplying reliable water to the Phoenix Basin. Only once or twice in most people’s lifetimes would there have been a noteworthy flood or drought. Canals and settlements stretched toward the horizons.

Then came the change. Donald Graybill, a researcher at the University of Arizona in Tucson, has been piecing together the flow record of the Salt River through tree rings and layers of water deposit. He found that at the onset of the Hohokam Classic Period, around A.D. 1190 — that is, less than 1,000 years ago — weather became erratic, and the Salt River started flooding and dwindling at twice its previous rate, blowing out canals and then leaving them dry. These flood and drought events were clustered, hitting farmers year after year. Mortality rates immediately began rising.

It wasn’t just the Salt River that had trouble. All of the Southwest fell prey. The cultural landscape of the Southwest mobilized. Migrants looked for new beginnings wherever they could, displacing outlying communities, forcing masses of people into the Phoenix Basin. Populations swiftly doubled at some Hohokam sites. They all needed to be fed, and more important, they all needed water.

While migrants poured in, communities reorganized, now gathering around newly built platforms, planks of land about 10 feet tall, upon which walled-in, multistory architecture was erected. The first gated communities appeared. People were shoring themselves up, as if building castles.

Archaeologists have looked at where pottery was being made and shipped and found that people no longer traded with specialists, as they once had. Instead of relying on a centuries-old intra-Phoenix exchange network, people began making their own pottery, isolating themselves. The famously artistic tradition of red-on-buff ceramics ended, overwhelmed by an influx of polychrome styles more representative of migrant populations. Meanwhile, new canals were being built higher in the drainage area, depriving downstream users of what little water was available.

Disease and malnutrition spread. At first, women and children took much of the brunt, but men ultimately perished at the same increased rate. Excavations of burials at the largest settlement in the city, now near today’s airport, revealed that during this period 66 out of every thousand people died each year. (Between eight and nine out of every 1,000 U.S. residents die each year, according to government statistics.)

At a dig site in downtown Tempe, a suburb surrounded by Phoenix, 176 burials were recently unearthed from beneath a parking lot. One was a 15-year-old girl with osteoporosis and scoliosis, her ribs painfully deformed, spine bending like a river. Her burial had been richly furnished with goods dating to the Hohokam decline.

Originally, archaeologists thought they would just go in for a cursory check, quickly giving the OK to start construction on a new transit center. Eight months later, they are still on their hands and knees, scraping up two acres of Hohokam rooms and buildings.

Eric Cox, leader of the dig, walked me through his site while traffic rumbled past; ruins stood ankle-high around us, chopped down by centuries of wear and progress. Cox was trying to close the excavation and make way for a new light-rail station, but his crews kept finding more burials, and by law they had to dig until all were accounted for.

During the drought, Cox said, the Hohokam were burying people under rooms and beneath courtyards, some stacked on top of each other to accommodate the number of bodies. Infant mortality rates had doubled.

“In this area we had mostly women and infants,” Cox said, raising his voice as one passenger jet after the next climbed over his head. “We had a lot of young women. One was actually buried with a neonatal skull; looked like she possibly had a miscarriage, and it killed them both. This house over here, we had a female buried with two infants, and another infant buried in the floor after this place was abandoned, and another two just north of it.”

The last Hohokam settlements were abandoned some time in the 15th century, and these were merely shanties that had been built within the ruins of 14th century compounds. The final collapse involved a few people struggling to survive; everyone else had left, searching for homes elsewhere in the Southwest.

Like many archaeologists, Cox talks about Hohokam settlement and modern Phoenix in the same breath. Looking across his dig site, an upended graveyard of women and children, he said, “I think we’re going to carrying capacity, and then basically we’re going bust. I look around at everything we’re building, and all the people moving in. I don’t see how we can sustain it.”