Megafloods and the Collapse of Cahokia

My first understanding of Cahokia and pre-Columbian cities of the Americas came from the book 1491. Between 1050 and 1200 A.D., Cahokia was North America’s largest and most prominent cultural center north of Mexico. It wielded economic power and religious influence from the Great Lakes to the Gulf of Mexico. A new paper published in PNAS describes a possible mechanism for its collapse; flooding.

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)

The authors stumbled upon this finding. They were extracting deep cores of sediments from two sites in the Mississippi River floodplain, nearby Cahokia’s former boundaries, each cross-section representing 1,800 years of geologic history. When examining these cores they noticed unusual layers with a fine and uniform texture, without pollen, charcoal or fossils… These sterile layers with typical of flood water sediments. It turns out these layers dated to the same time we saw the decline of Cahokia culture. Does that mean floods contributed to the downfall of this great city?