New research published in the Proceedings of the National Academy of Sciences shows that Cahokia – the largest prehistoric settlement in the Americas north of Mexico – emerged during a period of the reduced frequency of large floods in the Mississippi River valley, and that its decline and abandonment followed the return of megafloods.
A map showing approximate areas of various Mississippian and related cultures. Cahokia is located near the center of this map in the upper part of the Middle Mississippi area
Best known for large, man-made earthen structures, the pre-Columbian city of Cahokia was inhabited from about 600 to 1400 CE. Its emergence as a regional center can be traced to the population growth and intensified cultivation of native domesticated plants that began around 400 CE in the floodplain of the central Mississippi River. By 1050 CE, Cahokia emerged as a hierarchically organized cultural and political center in this region. By 1250, the city’s population rivaled Paris and London; at its peak in 1300, Cahokia numbered an estimated 40,000 people.
The Cahokians were advanced people who did not appear to be related to any major known Native American tribes. They were accomplished builders who erected a wide variety of structures from practical homes for everyday living to monumental public works that have maintained their grandeur for centuries.
Cahokia’s baseline transects Woodhenge, Monk’s Mound, and several other large mounds on the city’s east-west axis. The Grand Plaza is a large open plaza that spreads out to the south of Monks Mound. Researchers originally thought the flat, open terrain in this area reflected Cahokia’s location on the Mississippi’s alluvial flood plain but instead soil studies have shown that the landscape was originally undulating. In one of the earliest large-scale construction projects, the site had been expertly and deliberately leveled and filled by the city’s inhabitants. It is part of the sophisticated engineering displayed throughout the site. The Grand Plaza covered roughly 50 acres (20 ha) and measured over 1,600 ft (490 m) in length by over 900 ft (270 m) in width. It was used for large ceremonies and gatherings, as well as for ritual games, such as chunkey. Along with the Grand Plaza to the south, three other very large plazas surround Monks Mound in the cardinal directions to the east, west, and north.
The fate of the Cahokians and their once-impressive city has been mysterious until now.
A new study, led by Samuel Munoz of the University of Wisconsin-Madison, suggests that major flood events are tied to the cultural center’s emergence and ultimately, to its decline. Munoz and his colleagues went to Horseshoe Lake, near the 6-sq-mile city’s center, and collected cores of lake mud to look for pollen and other fossils that document environmental change.
“We had these really strange layers in the core that didn’t have any pollen and they had a really odd texture. In fact, one of the students working with us called it lake butter,” Munoz said. The scientists used radiocarbon dating of plant remains and charcoal within the core to create a timeline extending back nearly 2,000 years.
In so doing, they established a record of eight major flood events at Horseshoe Lake during this time, including the fingerprint left by a known major flood in 1844. To validate the findings, the archaeologists also collected sediments from Grassy Lake, roughly 120 miles downstream from Cahokia, and found the same flood signatures.
The new findings show that floods were common in the region between 300 and 600 CE. 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 diminished and the climate became more arid, archaeological evidence shows that people had moved down into the floodplain, began to increase in population, and farmed more intensively.
“While the region saw frequent flood events before 600 CE and after 1200 CE, Cahokia rose to prominence during a relatively arid and flood-free period and flourished in the years before a major flood in 1200,” the scientists said.
“We are not arguing against the role of drought in Cahokia’s decline but this presents another piece of information,” Munoz said.
“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 study senior author Prof John W. Williams of the University of Wisconsin-Madison.
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