Did Megafloods spell the end of the Ancient City of Cahokia?

The mysterious demise of the ancient city of Cahokia has long remained unexplained, but recent research suggests catastrophic megafloods may have devastated crops and food stores and forced residents to suddenly abandon their city some 700 years ago.

A reconstruction of Cahokia with Monks Mound in the distance.

Once North America’s largest and most sophisticated cultural center north of Mexico, the ancient city of Cahokia, located in present-day Southern Illinois, was an economic powerhouse at its height from about 1000 to 1150 AD. 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 6 square miles, boasting 120 man-made mounds — the largest of which was 10 stories or 100 feet in height, and its foot-print covered 14 acres or 610,000 square feet. Known as Monks Mound, it was the largest prehistoric earthwork in the Americas. The building of the mound was a massive undertaking, requiring an estimated 79,000 square feet of earth. 

Samuel Munoz, geographer at the University of Wisconsin-Madison, has spent a number of years researching how Cahokia’s residents shaped the local landscape, such as how farming affected the region. During this research he discovered the buried remains of a massive flood dated that likely destroyed the crops and houses of more than 15,000 people. Evidence for the flood is a silty deep core sediment layer nearly 8 inches thick, dated to 1200 AD, +/- 80 years. Although Cahokia wasn’t completely abandoned until 1350 AD, Munoz had been able to show that the area experienced periods of severe flooding as the climate changed over the centuries. The catastrophic flood would have shaken the confidence of the city, eventually leading them to make the decision to move on and the inhabitants never returned.

Beginning of the End
The exact reasons behind the city’s decline had 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.

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

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

Megafloods
Munoz described the cycle which doomed the city of Cahokia, saying, “Beginning around 600 AD, high-magnitude floods became less frequent, and indigenous peoples moved into the floodplain and began to farm more intensively and increased their numbers.”

Around 1200 AD, +/- 80 years, 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. Floodwaters, which are thought to have risen 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.

Archaeologist George Milner at Pennsylvania State University 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. The real problem starts when indigenous people experience back-to-back failures.

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 Valley region.

Compiled by Neil Gale, Ph.D.