An illustration of North America's first city, Cahokia.

The Rise and Fall of Cahokia: Did Megafloods Spell the End of the Ancient Metropolis?

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The mysterious demise of the ancient city of Cahokia has long remained unexplained, but now research suggests catastrophic megafloods may have devastated crops and food stores, and forced residents to suddenly abandon the metropolis some 800 years ago.

Archaeology news site Western Digs reports that Sam Munoz, geographer at the University of Wisconsin has been leading research into the area’s agricultural and geological history. Through examination of deep core sediment samples from the Mississippi River flood plain and surrounds, he and his team have been able to show that the area experienced periods of severe flooding as the climate changed over the centuries.

Interpretive Center at Cahokia Mounds, display depicting everyday life in the once-thriving ancient metropolis.

Interpretive Center at Cahokia Mounds, display depicting everyday life in the once-thriving ancient metropolis. Daniel X. O’Neil/ Flickr

Once North America’s largest and most sophisticated cultural center north of Mexico, the ancient metropolis of Cahokia, located in present day Illinois in the United States, was an economic powerhouse at its height (circa 1050 to 1200 A.D.) Its sphere of political and religious influence extended from the Great Lakes to the Gulf of Mexico. The city was home to approximately 20,000 people and it sprawled over nearly 1,600 hectares, boasting 120 man-made mounds— the largest of which was a ten-story earthen colossus known as Monk’s Mound, the largest prehistoric earthwork in the Americas. The building of the mound was a massive undertaking, requiring an estimated 22 million cubic feet of earth. However, by the early 13th century the city was abandoned without explanation, and the inhabitants never returned.

Monks Mound, Cahokia site, 1887 illustration.

Monks Mound, Cahokia site, 1887 illustration. Public Domain

Beginning of the End

The exact reasons behind the city’s decline has long been debated by scientists. Various theories include political battles, crop failures, climate change and an epic fire . However, Munoz and colleagues were able to establish the timing and severity of the historical flood patterns in the area.

Cahokia’s decline coincided with a major Mississippi River surge, around 1200 A.D.

The sediment core samples contained almost no charcoal, pollen, or plant matter fossils, and instead were made up of silty clay, much like floodwater sediments. This indicated a period of flooding. However, the layers above and below the clay contained the telltale markers of aridity, such as plant material and charcoal. Researchers were able to date the various samples and create a timeline of events.

A mural depicting the ancient city of Cahokia.

A mural depicting the ancient city of Cahokia. Kmaschke/ Flickr

Megafloods

Munoz described the cycle which doomed the city of Cahokia, saying, “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.”

Around 1200, however, the North American climate became wetter and the waters rose, flooding the area with severe and frequent deluges. Crops would have suffered, food stores were probably ruined, and the population would have had to relocate or starve.

Professor of anthropology Sissel Schroeder accompanied the research team. He told news site IBTimes that the floodwaters, which are thought have risen 10 meters (33 feet) above base elevation, would have jarred a population unprepared for such environmental challenges.

“It would have had a particularly destabilizing effect after hundreds of years without large floods," Schroeder said.

Archaeologist George Milner at Pennsylvania State University who was not involved in the study, found the analysis convincing, but suggested that megafloods might have been only one of many catastrophes that eventually led to Cahokia’s downfall, including droughts, fires, cold and hot years—all leading to social instability.

Milner told science journal Nature, “The real problem of these kinds of societies is when people experience back-to-back failures.”

Study results have been published in the Proceedings of the National Academy of Sciences . The findings by Munoz and colleagues may have finally solved the mystery of the abandoned city of Cahokia, and potentially given us a glimpse into what the future might hold for the flood-prone Mississippi River region.

Featured Image: An illustration of North America's first city, Cahokia. Image source .

By Liz Leafloor

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