Nearly 1,000 years ago, a bustling city sprouted in southern Illinois. As many as 20,000 people lived in this six-square-mile metropolis located near modern St. Louis around the year 1050, but then the civilization simply disappeared. Now, one team of researchers thinks it knows why.
An academic study published this month reveals that the city now called Cahokia Mounds may have been abandoned because of increased flooding of the Mississippi River. The study provides a new theory for why the ancient society quickly dissolved.
Cahokia Mounds is an archaeological site located near Collinsville, about six miles east of St. Louis. At its peak, it was likely the largest city in what is now the United States, and its size as a North American archaeological site is surpassed only by the Aztec ruins of central Mexico.
The site is named for the Cahokia subtribe of the Illiniwek native peoples, and it’s unclear what the city would have been called by its original inhabitants. It would have rivaled the size of many European cities at the time and even had a defensive wall 15 feet tall. However, the city was abandoned by the year 1400, well before the arrival of white settlers in America. Cahokia Mounds was designated a UNESCO World Heritage Site in 1982.
Samuel Munoz, a doctoral student at the University of Wisconsin-Madison, first became interested in Cahokia Mounds because of its importance in the “prehistory” of America – the time before white explorers and settlers began to record events here.
From 2011 to 2013, Munoz led a research team which extracted columns of sediment from nearby Horseshoe Lake, an oxbow lake that became isolated from the Mississippi River about 1,700 years ago. The researchers also extracted similar samples from a lake 100 miles downriver. When the team analyzed the sediment cores, both sites confirmed a pattern of dry weather followed by wet weather that may explain Cahokia’s decline.
Munoz says sediment cores from lake beds are useful because they serve as a record of events at a given location.
“There’s lots of material accumulating over time, so lakes can be considered the trash heaps for the environment,” he said. “Things get piled on, and it reflects what’s happening over time.”
The texture of flood sediment deposits is different from that of normal deposits, Munoz says. Flood deposits are lighter in color and have a very fine texture – so fine that one member of the research team nicknamed the flood sediment “lake butter.” The age of the sediments in each sample was determined by radiocarbon dating.
Munoz says the rise of Cahokia coincides with a dry period beginning around the year 600, which is reflected in the lack of flood sediments from that period. The river didn’t have a significant flood between the approximate years 600 and 1200, allowing the farming of large swaths of land. Pollen samples in the sediment cores revealed deforestation and increased cultivation of corn during that time, which Munoz says would indicate the clearing of land for farming.
However, the lakebed samples also show a 7.5-inch layer of sediment deposited by a flood around the year 1200. The flood sediment marks a significant decrease in the amount of pollen, indicating that farming at Cahokia almost completely collapsed after the flooding. The dry period and subsequent wet period recorded in the sediment samples match what scientists already know about America’s climate history, Munoz says.
The flood around the year 1200 had to be at least 33 feet high, Munoz says, and it likely surprised the residents of Cahokia.
“They had been living there for 600 years and no big flood like that had ever come,” he said. “Their farming and political institutions were probably not very well prepared for that.”
Munoz believes the return of flooding caused or at least contributed to the abandonment of Cahokia. He cautions that his team’s research doesn’t invalidate other explanations for Cahokia’s decline, like war or sickness.
“What we’re doing is providing evidence for another idea,” he said. “This doesn’t necessarily refute those other ideas; it just adds to the broader story.”
The study was published in the May 2015 issue of the journal Proceedings of the National Academy of Sciences.
Munoz says the fate of Cahokia Mounds can be a lesson for the modern world.
“It really shows how dependent we are on the Mississippi and other rivers, and how sensitive rivers are to climate change,” he said. “We might forget that sometimes, but we need to keep a close eye on rivers and know how they’re going to react as the climate changes.”
A separate study published in 2012 showed that pottery vessels found at Cahokia Mounds contained traces of a plant used to make a ceremonial concoction known as “black drink.” The drink, made with a holly leaf that only grows in the southeastern U.S., contained caffeine and was used to make the drinker vomit as part of a purification ritual. The discovery at Cahokia meant that black drink was consumed by ancient native peoples much earlier than was previously proven, and it pointed to the existence of a vast trade network between different regions and tribes.
Contact Patrick Yeagle at firstname.lastname@example.org.