Researchers studying sediment cores from lakes along the Mississippi River have found a history of flooding, which could help to explain the decline of an early Native American civilization.
From roughly 600 to 1400 CE, the largest city in what is now the United States was Cahokia. At its height, long before the arrival of Europeans, the city covered six square miles and consisted of 120 human made mounds that served a variety of functions including habitation, religion and commerce.
It is thought that at the city’s peak it’s population numbered about 40,000. That would remain a record for any city north of Mexico until 1780 when Philadelphia reached 40,000.
A number of different causes have been suggested for the city’s decline, which began in about 1200 CE. These include erosion, drought, over-hunting and deforestation.
Now, a new study published in the Proceedings of the National Academy of Sciences has added another possible cause to the list, flooding. Using sediment cores, the researchers found a long history of flooding in the area which diminished considerably about 600 CE, roughly the time the city was founded, are resumed around 1200 CE, which coincides with the start of the city’s decline.
Studying sediments which date back almost 2,000 years, the researchers found eight significant flood events in the valley near modern day St. Louis. The researchers do not indicate flooding as a sole cause of the demise of Cahokia, merely another factor to be considered.
“We are not arguing against the role of drought in Cahokia’s decline but this presents another piece of information,” says Samuel Munoz, a Ph.D. candidate in geography and the study’s lead author in a statement.
The authors also believe that the findings could provide useful information for modern residents of the area.
“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,” said Jack Williams, a professor of geography and director of the Nelson Institute for Environmental Studies Center for Climatic Research.
Munoz was originally looking for information on prehistoric land use among ancient people. The researchers took core samples from Horseshoe Lake, which is near the ancient city center of Cahokia to look for pollen and other samples which would provide information on ancient land use.
“We had these really strange layers in the core that didn’t have any pollen and they had a really odd texture,” Munoz says. “In fact, one of the students working with us called it ‘lake butter.'”
They asked around to try to determine what they had found and the the late Jim Knox, who spent 43 years as a geography professor at the University of Wisconsin-Madison, suggested that they think about flooding. Large floods can significantly disrupt the normal depositing of materials in lake beds.
The team radiocarbon dated charcoal and plant material within the core to create a record of flood events, including a known flood in 1844. Next they validated these findings by comparing them to sediments from Grassy Lake, 120 miles downstream. The sediments and flood “signatures” from the second lake agreed with their earlier findings and matched the timeline.
The findings show regular flooding in the region between 300 and 600 AD. Evidence in the sediments shows that agriculture first began in the region about 400 CE and that by 600 the region had become more arid and significant numbers of farmers had moved into the floodplain.
“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.
Sissel Schroeder, UW-Madison professor of anthropology whose doctoral studies focused on Cahokia said that evidence from the city does not suggest flooding, but that it can’t be ruled out. Researchers may have simply missed the signs of a flood.
Archologists know that about 900 the population exploded after farmers began to cultivate maize in the region. The city is generally viewed as a chiefdom with a hierarchy of settlements spreading from the main city. This is similar, says Schroeder, to small county seats surrounding large centers that exist today. Around 1200 however, the population began to decline.
“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. 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,” says Schroeder.
By 1400 the city was deserted and its people had scattered to other parts of North America.
According to the researchers, in order to replicate the flooding of the time, the Mississippi River would have to rise about 33 feet above its current levels at St. Louis. Flooding of this severity at the time would have devastated local crops and severely impacted food stores creating widespread shortages.
“We hope archaeologists can start integrating these flood records into their ideas of what happened at Cahokia and check for evidence of flooding,” says Munoz, who plans to continue studying flood records in lakes around the country once he graduates this year.
The Cahokia Mounds State Historic Site is in Illinois, directly across the Mississippi River from St. Louis, Missouri. The 2,200 acres park contains about 80 mounds. It is listed as a National Historic Landmark, a UNESCO World Heritage Site and is designated for state protection.
Additional multimedia information about the site and its inhabitants is available at cahokiamounds.org, an airial tour of the site along with additional history can be found at Chickasaw.tv and a narrated video about the site can be found on the IHPA’s YouTube channel.
