|A reconstruction of Cahokia with Monks Mound in the distance.|
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.|
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.