No one knows for sure why Cahokia, the largest Native American settlement north of Mexico, fell.

Cahokia was first settled around A.D. 600. The city and areas around it prospered during a flood-free period from A.D. 600-1200. Archaeologists estimate Cahokia’s population was as high as 40,000 during its peak.

If that 40,000 estimate is correct, one of Cahokia’s biggest issues would have been supplying food for all of its inhabitants.

The most widely accepted theory for Cahokia’s fall is drought, along with political instability. New research suggests major floods along the Mississippi River valley could be responsible for Cahokia’s rise and eventual fall.

Cahokia painting

Credit: Painting of what Cahokia may have looked like in 1150, Michael Hampshire

A research team led by University of Wisconsin-Madison geographers Samuel Munoz and Jack Williams point to evidence from beneath two lakes in the Mississippi floodplain.

Researchers found eight flood events dating back 2,000 years in sediment cores. Does this mean the drought theory gets tossed out the window?

“We are not arguing against the role of drought in Cahokia’s decline but this presents another piece of information,” says Munoz.

The researchers didn’t set out to investigate previous floods along the Mississippi River. The original research was tied to prehistoric land use on ancient forests. The new findings were “kind of an accident,” Munoz says.

Initially, Munoz and his fellow researchers were collecting sediment cores to look for pollen or other signs of environmental change. It works just like a tree core. Each layer tells a story.

“We had these really strange layers in the core that didn’t have any pollen and they had a really odd texture,” Munoz says. “In fact, one of the students working with us called it ‘lake butter.’”

The “strange layers” turned out to be tied to flooding. The researchers confirmed the find by gathering samples at another lake downstream from Cahokia and found the same layers.

The sediment cores show flooding was common between A.D. 300 – 600. By 600, the flooding had stopped. People began to settle and farm the floodplain.

But, around 1200 – changes become apparent.

Sissel Schroeder, a professor of anthropology, highlights some of these changes.

“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,” says Schroeder. “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.”

The sediment cores also show a major flood event right around this time.

200 years later, Cahokia was deserted. We will never know the exact reason for Cahokia’s fall, but a major flood event certainly would not have helped. Still, 600 years is impressive for a city at a time when political instability was the norm.

Jack Williams, a professor of geography, touched on what these findings mean for cities today. “It also provides new information about the flood history of the Mississippi River, which may be useful to agencies and townships interested in reducing the exposure of current landowners and townships to flood risk.”

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