Major floods in the Mississippi river have been linked to both the rise and fall of the ancient Cahokia civilization.
This region, which is north of Mexico, was home to one of the largest prehistoric settlements in the Americas, the University of Wisconsin-Madison reported. Two-thousand-year-old Sediment cores hidden beneath two lakes on Mississippi's flood plain showed evidence of eight major historical floods in the Mississippi River valley that may explain the emergence and demise of Cahokia.
Cahokia was in its prime during a relatively dry and flood-free period, and thrived until a devastating flood occurred in 1200 A.D.; the region saw frequent flooding before and in the early days of the peoples' rise. Two-hundred years after this major flood, the nation was almost completely wiped out.
"We are not arguing against the role of drought in Cahokia's decline but this presents another piece of information," said Samuel Munoz, a Ph.D. candidate in geography and the study's lead author.
"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 Jack Williams, a professor of geography and director of the Nelson Institute for Environmental Studies Center for Climatic Research.
The sediment cores looked at in the study first caught the researchers' attention because they showed an unusual lack of pollen and curious texture, leading to their nickname "lake butter." The late researcher Jim Knox suggested flooding could lead to these strange sediment core properties.
The researchers used radiocarbon dating of plant remains and charcoal found in the core to create a timeline of past flooding events. They identified eight major floods in Horseshoe Lake over the past two millennia, including one which is known to have occurred in 1844. To back up their findings, the team also looked at sediment cores from Grassy Lake, which is about 120 miles downstream form Cahokia, and identified the same flood signatures.
The findings suggest floods were frequent in the region between 300 and 600 A. D; the first evidence of agricultural settlements in Cahokia have been dated back to about the year 400. When the area became drier around the year 600, the population moved into the floodplain and rapidly multiplied. Around the year 900 the civilization started to cultivate maize, causing their population to "explode." Around 1200 the populations started to decline, and this is around the same time as a major flood was shown to have occurred.
"We see some important changes in the archaeology of the site at this time, including a wooden wall that is built around the central precinct of Cahokia," said Sissel Schroeder, a UW-Madison professor of anthropology whose doctoral studies focused on the Cahokia area. "There are shifts in craft production, house size and shape, and other signals in material production that indicate political, social and economic changes that may be associated with social unrest."
Around this time, the Cahokia people are believed to have started migrating to other areas of North America, and by 1400 the area was deserted. The researchers note other factors most likely contributed to the fall of the Cahokia, but a major flooding event may have been the "proverbial last straw."
"We hope archaeologists can start integrating these flood records into their ideas of what happened at Cahokia and check for evidence of flooding," Munoz said.
The findings could also have implications for the prediction of future Mississippi flooding events. The historical data suggests these major floods occurred about once every century or two before European settlement and intervention.
"We have managed the river so much to prevent floods from happening, we don't have a good baseline for how the river behaves without human modification," Munoz said. "This may help us understand not only how it once behaved, but how it may behave in the future."
The findings were published in a recent edition of the journal Proceedings of the National Academy of Sciences.
