‘Megafloods’ Spurred Collapse of Ancient City of Cahokia, New Study Finds

It’s been attributed to war, crop failures, political strife, and even an epic fire. But new research in the heart of one of North America’s most influential prehistoric cultures suggests that its demise may have been brought about, at least in part, by disastrous “megafloods.”

The new findings come from a team of archaeologists and earth scientists who have been studying the land surrounding the ancient metropolis of Cahokia in what’s now southern Illinois.

At its peak — between around 1050 and 1200 — Cahokia was the continent’s largest and most prominent cultural center north of Mexico, wielding economic power and religious influence from the Great Lakes to the Gulf of Mexico.

But by the early 13th century, signs of the city’s might began to wane as suddenly and mysteriously as they had first appeared. And while many human factors likely played a role — including economic hard times and bloody conflicts — researchers say one important force remained out of Cahokians’ control: the Mississippi River. [Learn about human sacrifice at Cahokia: “Infamous Mass Grave of Young Women in Ancient City of Cahokia Also Holds Men: Study“]

“Our study shows that Cahokia emerged during an unusually arid period in midcontinental North America, when large floods were suppressed,” said Sam Munoz, a geographer at the University of Wisconsin, who led the current research.

“But [it] began to decline after A.D. 1200, when large floods became more frequent.”

Cahokia illustration
An artist’s depiction shows central Cahokia around the year 1150 (Cahokia Mounds Museum Society/ Art Grossman)

Munoz and his colleagues made this discovery somewhat by accident, while researching the area’s agricultural history.

They extracted 10 deep cores of sediments from two sites in the Mississippi River floodplain around Cahokia’s former boundaries, each cross-section representing 1,800 years of geologic history.

“When we were examining these cores, we noticed unusual layers that had a very fine and uniform texture, and contained almost no pollen, charcoal, or plant macro fossils,” Munoz said.

These strangely sterile layers turned out to contain the silty, small-grained clay typical of floodwater sediments.

“We designed this study so that if we saw the same kind of deposits in both [sets of] cores at the same times, then that would confirm that they were deposits from floods of the Mississippi River,” Munoz said.

“We found five out of five overlapping deposits with distinct particle-size distributions that were deposited at the same times, and thus concluded that these must represent Mississippi River floods.”

The layers above and below those bands of flood sediment, meanwhile, contained charcoal and other plant material that allowed the researchers to date the strata, and re-create Cahokia’s environmental past.

Until about 1,400 years ago, the evidence showed, the area where the ancient city would one day stand was prone to frequent and severe floods, with the Mississippi River rising at least 10 meters (about 33 feet) above its base level.

But then the climate shifted, and the great floods stopped.

“Beginning around A.D. 600, high-magnitude floods became less frequent, and indigenous peoples moved into the floodplain and began to farm more intensively and increase their numbers,” Munoz said.

By the mid-11th century, these settlements had grown into a metropolis that, at its zenith, housed at least 10,000 people in its central district.

However, Munoz’ team found that the city only continued to thrive at the pleasure of the Mississippi.

Starting around 1200, the data show, the climate of central North America became wetter again, and the large floods returned, inundating the region with increasing frequency. Munoz’ team speculates that these “megafloods” would have devastated crops, ruined caches of food, and forced the temporary relocation of thousands of people.

While there’s no direct archaeological evidence of the disruptions that these disasters likely caused on Cahokians’ lives, Dr. Sissel Schroeder, a Wisconsin archaeologist who collaborated in the research, said that the return of the floods coincides closely with many signs of political instability and social upheaval in the community.

“We see some important changes in the archaeology of the site at this time, including a wooden wall that is built around the central precinct of Cahokia,” Schroeder said, in a press statement.

Population in the region soon began to drop, as well as agricultural production.

Then, the construction of religious and elite structures that helped hold the community together came to a halt. [Read about Cahokia’s ceremonial beverage: “Ancient Americans Pounded Vomit-Causing ‘Black Drink’ 6 Times Stronger Than Coffee“]

cahokia flood map
A map shows the central district of Cahokia (inset right) in the context of the water levels reached by the flood of 1844 (blue). (Credit: Sam Munoz)

“There are shifts in craft production, house size and shape, and other signals in material production that indicate political, social, and economic changes that may be associated with social unrest,” Schroeder said.

“It would have had a particularly destabilizing effect after hundreds of years without large floods.” [Explore other recent research into Cahokia’s collapse: “Epic Fire Marked ‘Beginning of the End’ for Ancient Culture of Cahokia, New Digs Suggest” ]

The new clues found in the ground under Cahokia may reflect only one of many contributors to the ancient city’s collapse, Munoz said.

But they may still hold lessons for modern societies, as they too grapple with a changing climate.

“Beyond the Cahokia site, our results demonstrate how sensitive large rivers like the Mississippi are to climatic variability — and how dependent human societies are on rivers.

“It isn’t clear yet how rivers like the Mississippi will respond to the climatic changes projected for the 21st century, because our historical records cover only the last 100 to 150 years, and do not represent the range of climatic variability projected for the next 100 years.”

Munoz and his colleagues report their findings in the Proceedings of the National Academy of Sciences.

Munoz SE, Gruley KE, Massie A, Fike DA, Schroeder S, & Williams JW (2015). Cahokia’s emergence and decline coincided with shifts of flood frequency on the Mississippi River. Proceedings of the National Academy of Sciences of the United States of America PMID: 25941363

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