Massive flooding in the Mississippi River Valley may have been the last straw in the demise of the largest prehistoric settlement in the Americas north of Mexico, according to a new study.

The pre-Columbian city of Cahokia – located near what is now St. Louis – was inhabited from about 600 to 1400 A.D. Researchers have put forward a number of possible causes for its disappearance, from drought to social upheavals.

Cahokia was a cultural and political center of the region, inhabited by an advanced people apparently unrelated to any other major known Native American tribe.

A new study led by University of Wisconsin-Madison researchers suggests major flood events in the Mississippi River Valley may be linked to both the rise of the city that reached a population of 40,000 and its eventual demise.

Sediment cores taken from two lakes on the river's flood plain show evidence of frequent flooding before 600 A.D. and after 1200 A.D. It was between those years – a relatively dry and flood-free period – that Cahokia rose to prominence.

After 1200 A.D., the 6-square-mile city of large earthen structures began to experience political instability and a decline in its population. By 1400 A.D. it was completely abandoned.

While drought has long been considered the most likely cause – the reason for the disappearance of a number of early agricultural societies in North America – new findings suggest another possibility: too much water.

"We are not arguing against the role of drought in Cahokia's decline, but this presents another piece of information," said Samuel Munoz, a doctoral candidate in geography and lead author of a study appearing in the Proceedings of the National Academy of Sciences.

The lake sediments studied by the researchers provide compelling evidence that massive flood events may have been the tipping point for the ancient people of Cahokia.

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

The floods would have to have been massive — with flood stages possibly 30 feet above the Mississippi's base level.

Floods of this magnitude could have destroyed the region's crops, reduced essential food stores and created agricultural shortages, the researchers explained — perhaps pushing the Cahokia civilization past the point at which it could still survive.

"It would have had a particularly destabilizing effect after hundreds of years without large floods," researcher Sissel Schroeder concluded.

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