Massive floods in the Mississippi River valley may have wiped out an ancient civilization, according to a new study.

Cahokia were the largest prehistoric settlement in the Americas north of Mexico until the year 1200 AD, when the once thriving population began to decline, ultimately disappearing by 1400. Many factors have been blamed for their decline - from extreme droughts to social unrest - but new findings suggest the rise and fall of rivers contributed to the ultimate downfall of the Cahokia civilization.

"We are not arguing against the role of drought in Cahokia's decline but this presents another piece of information," lead author Samuel Munoz of the University of Wisconsin-Madison said in a statement.

"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," added geographer and co-author Jack Williams.

The researchers collected sediment core samples hidden beneath two lakes in the Mississippi floodplain, Horseshoe Lake and Grassy Lake - the latter located roughly 120 miles downstream from Cahokia. Dating back nearly 2,000 years, the samples provided evidence of at least eight major flood events in the central Mississippi River valley. This may finally help explain the mysterious decline of Cahokia, near present-day St. Louis.

While the region saw frequent flood events before 600 AD and after 1200 AD, Cahokia rose to fame during a relatively arid and flood-free period and flourished in the years before a major flood in 1200, according to the study. But just 200 years later, Cahokia was completely abandoned.

Drought has been a popular theory behind their decline, as well as the decline of many other early agricultural societies in North America and the rest of the world. But the new sediment samples - which can capture and record past environment changes - hint that massive floods may have been the tipping point for these ancient people.

"Rarely do you get such fortuitous opportunities where you have these nice sedimentary records next to an archaeological site that's so well studied," said Munoz. (Scroll to read on...)

They show that floods were common in the region between 300 and 600 AD. Meanwhile, the earliest evidence of more agricultural settlement appears along the higher elevation slopes at the edge of the central Mississippi River floodplain around the year 400. But by 600, when flooding subsided and the climate became more arid, archaeological evidence shows that people had moved down into the floodplain and begun to increase in population and farm more.

In the past, little archaeological evidence suggests flooding at Cahokia, but researchers say it can't be ruled out completely. A major flooding event could have been the last straw.

"It would have had a particularly destabilizing effect after hundreds of years without large floods," explained Sissel Schroeder, one of the researchers.

And these floods would have had to be quite massive in order to deposit sediments into Horseshoe and Grassy Lakes. The Mississippi River would have had to rise 10 meters (about 33 feet) above its base elevation at St. Louis. This would have destroyed the region's crops, impacted essential food stores, and created agricultural shortages - all important things in a civilization like Cahokia.

"We hope archaeologists can start integrating these flood records into their ideas of what happened at Cahokia and check for evidence of flooding," Munoz concluded.

The study also provides new information about the river's past behavior in the central Mississippi Valley, and may help scientists understand how it may behave in the future.

The findings were published in the journal PNAS.

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