Why was the ancient city of Cahokia abandoned? New clues rule out one theory.

Glenn Hodges

NATIONAL GEOGRAPHIC

4/12/2021


About a thousand years ago, a city grew in the floodplain known as the American Bottom, just east of what is now St. Louis in Illinois. In a matter of decades, it became the continent’s largest population center north of Mexico, with perhaps 15,000 people in the city proper and twice as many people in surrounding areas. A couple centuries after its birth it went into decline, and by 1400 it was deserted.

© Photograph by Ira Block, Nat Geo Image Collection Cahokia's central plaza is now part of a 2,200-acre historical site.

The story of Cahokia has mystified archaeologists ever since they laid eyes on its earthen mounds—scores of them, including a 10-story platform mound that until 1867 was the tallest manmade structure in the United States. They don’t know why Cahokia formed, why it grew so powerful, or why its residents migrated away, leaving it to collapse. Hypotheses are abundant, but data are scarce.


Now an archaeologist has likely ruled out one hypothesis for Cahokia’s demise: that flooding caused by the overharvesting of timber made the area increasingly uninhabitable. In a study published recently in the journal Geoarchaeology, Caitlin Rankin of the University of Illinois not only argues that the deforestation hypothesis is wrong, but also questions the very premise that Cahokia may have caused its own undoing with damaging environmental practices.

“Cahokia was the most densely populated area in North America prior to European contact,” she says. “Sometimes we think that big populations are the problem, but it’s not necessarily the population size. It’s how they’re managing and exploiting resources.”

Logical sense vs data

In 1993, two researchers from Southern Illinois University Edwardsville, Neal Lopinot and William Woods, suggested that perhaps Cahokia failed because of environmental degradation. They hypothesized that Cahokians had deforested the uplands to the east of the city, leading to erosion and flooding that would have diminished their agricultural yields and flooded residential areas.

Given the clear evidence that Cahokians had cut down thousands of trees for construction projects, the “wood-overuse hypothesis” was tenable. It fit the available data and made logical sense, and the archaeological community largely embraced it as a possible—or even likely—contributor to Cahokia’s decline. But little was done to test it.

In 2017, Rankin, then a doctoral student at Washington University in St Louis (where she’s now a research geoarchaeologist), began excavating near one of Cahokia’s mounds to evaluate environmental change related to flooding. She discovered something she hadn’t been expecting to find: clear evidence that there had been no recurrent flooding of the sort predicted by the wood-overuse hypothesis.

Her research showed that the soil on which the mound had been constructed was stable during the time of Cahokian occupation. The mound had been in a low-lying area near a creek that would likely have flooded according the wood-overuse hypothesis, but the soil showed no evidence of flood sediments.

Those results led Rankin to question the assumptions that led not just to that particular hypothesis, but to all the environmental narratives of Cahokia’s decline. The idea that societies fail because of resource depletion and environmental degradation—sometimes referred to as ecocide—has become a dominant explanatory tool in the last half century.

And the reason for that is clear: We do see that happening in past societies, and we fear that it is happening in our own. But our present environmental crisis might be inclining us to see environmental crises in every crevice of humanity’s past, Rankin says, whether they were actually there or not.

“The people who lived here in North America before the Europeans—they didn’t graze animals, and they didn’t intensively plow. We look at their agricultural system with this Western lens, when we need to consider Indigenous views and practices,” Rankin says.

A difference of worldview

Cahokians were part of what anthropologists call Mississippian culture—a broad diaspora of agricultural communities that stretched throughout the American Southeast between 800 and 1500 A.D. They cultivated corn and other crops, constructed earthen mounds, and at one point gathered into a highly concentrated urban population at Cahokia. Whether that was for political, religious, or economic reasons is unclear. But it’s not likely that they saw natural resources as commodities to be harvested for maximum private profit.

Cahokians cut a lot of trees—thousands of them were used to build what archaeologists believe were defensive fortifications—but that doesn’t mean they were treating them as fungible goods, or harvesting them in unsustainable ways, the way European-Americans often did. Maybe they were heedless of their environment and maybe they weren’t, Rankin says, but we certainly shouldn’t assume they were unless there’s evidence of it.

“Look at what happened with the bison,” Rankin says. Plains Indians hunted them sustainably. But “Europeans came in and shot all of them. That’s a Western mentality of resource exploitation—squeeze everything out of it that you can. Well that’s not how it was in these Indigenous cultures.”

Tim Pauketat, a leading Cahokia researcher and Rankin’s supervisor at the University of Illinois, agrees that the difference in cultural worldviews needs to be considered more seriously. “We’re moving away from a Western explanation—that they overused this or failed to do that—and instead we’re appreciating that they related to their environment in a different way.”

And that suggests that hypotheses for Cahokia’s decline and collapse are likely to become more complex. Tristram Kidder, an anthropologist at Washington University in St. Louis who chaired Rankin’s dissertation committee, says, “There is a tendency for people to want these monocausal explanations, because it makes it seem like there might be easy solutions to problems.”

Kidder teaches a class on climate change, and he says that’s a constant temptation, not just for the students but for himself—to try to master the problem by oversimplifying it. If Cahokians had just stopped cutting down trees, everything would have been fine. If we only started driving electric cars, everything will be fine. But the reality is much more complex than that, he says, and we have to grapple with that complexity.

Lopinot, one of the archaeologists who originally proposed the wood-overuse hypothesis in 1993, and who is now at Missouri State University, welcomes Rankin’s research. He knew at the time he presented his hypothesis that it was just a reasonable attempt to make sense of a mystery.

“Cahokia’s decline wasn’t something that happened overnight,” he says. “It was a slow demise. And we don’t know why people were leaving. It might have been a matter of political factionalization, or warfare, or drought, or disease—we just don’t know.”

There are clues. In later years, Cahokians built a stockade encircling central Cahokia, suggesting that inter-group warfare had become a problem. And there is preliminary data suggesting there may have been a major drought in the region that would have made food production challenging. But those clues still need to be investigated, researchers say.

“Archaeology is not like physics, where you can set up controlled experiments and get the answers you’re looking for,” Rankin says. You have to get out there and dig, and you never know what you are going to find.