Ancient Flooding Revealed by Pollen Records

November 2013


Most scientists agree that climate change is increasing the frequency of extreme weather events, like floods and droughts, in Wisconsin and the rest of the world. There are many recent examples of the devastation caused by such events: the 2011 floods in Australia and Brazil, or the 2013 floods in Colorado. But how can we learn about the impact of extreme weather on ancient populations that did not leave behind any written records?

A study by UW-Madison graduate student Samuel E. Munoz has uncovered some novel clues: Munoz found that the pollen deposits in sediment cores tell a compelling story about floods that occurred almost 1,000 years ago.

Clues found near the site of Cahokia (above) indicated that a flood occurred at the height of the city's population and power
Clues found near the site of Cahokia (above) indicated that a flood occurred at the height of the city’s population and power

Munoz is pursuing a Ph.D. in geography at UW-Madison and recently presented his work at the annual conference of the Geological Society of America. His area of expertise is the study of archaeological pollen records.

Noticing that few pollen studies had been conducted in the American Southeast, Munoz decided to investigate one of the largest prehistoric Native American settlements north of Mexico: the ancient city of Cahokia, located in the Illinois floodplain of the Mississippi River not far from modern St. Louis, Missouri.

Not only was Cahokia a very large settlement of the so-called Mississippian Culture, it was also surrounded by a great mystery: after a period of intense corn cultivation between 900 and 1200 C.E., which supported more than 15,000 Cahokia residents at its peak, agricultural production massively declined and essentially came to a complete halt by 1350 C.E. What could have caused this drastic change?

With funding from the National Geographic Society, Munoz set out to answer this question. He analyzed pollen records in sediment cores from Horseshoe Lake, which separated from the Mississippi River some 700 years before the city of Cahokia arose near its shores.

Horseshoe Lake is an example of a so-called oxbow lake: it started out as a curve, or meander, in the Mississippi River. As erosion and sediment deposition allowed the river to take a new shortcut through the low-lying land at the narrow end of the meander, the original curve became an independent stillwater lake. The rich soil between the river and the newly formed oxbow lake was well-suited for corn horticulture, which was the cornerstone of Mississippian civilizations like Cahokia.

Munoz’s unique finding in the sediment cores of Horseshoe Lake was a 7.5 inch layer of silty clay that separated the pollen records during the period of rapidly increasing corn cultivation up to 1200 C.E. from its drastic decline and virtually complete absence by 1350 C.E.

According to Munoz, the only possible explanation for this silty clay layer is a massive ancient flood. It is likely to have devastated both croplands and residential areas, contributing to, or even causing, a massive population decline during the next 150 years. By 1350 C.E., Cahokia had become a deserted ghost town whose only remaining structures were earthen mounds that had once been used for religious and political purposes.

The largest and most famous mound at Cahokia’s center is some ten stories high and was named Monk’s Mound by European settlers. It was apparently not destroyed by the mega-flood. Such massive mounds are a common feature of the Mississippian Culture settlements that can be found throughout the Mississippi River valley and its tributaries. The trading network of these Native American cultures, which flourished from the 10th to the 13th century, extended from the Great Lakes to the Gulf Coast.

A perfect example for a Wisconsin-based Mississippian culture is Aztalan State Park, located just south of the town of Aztalan, WI, along the Crawfish River. It was established in 1952 and is part of the National Register of Historic Places. The park includes three platform mounds and a reconstructed stockade. The stockade surrounded the Aztalan settlement on three sides as a vertical wall of logs set in the ground, supported by interwoven willow branches and a clay-grass plaster filling in the gaps.

As indicated by the pollen records analyzed by Munoz, the occurrence of extreme weather events like floods and droughts is not unique to present times. The main difference is that these events were few and far between, occurring more or less randomly throughout time, while climate change is likely responsible for their more frequent occurrence in the 21st century.

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