One morning in the spring of 2013, the usual crowd of picnickers and fishermen at Horseshoe Lake State Park in Pontoon Beach, Illinois were joined by a small group of scientists. Recreationists looked on as four fleece-clad geochemists loaded equipment into two small inflatable boats and ambled their way into the middle of the lake using wooden canoe paddles. At the helm was then University of Pittsburgh PhD candidate David Pompeani.
This was Pompeani’s second visit to Horseshoe Lake in two years. During the previous field season, he and fellow University of Pittsburgh graduate students had extracted a column of sediment from the lake bed. Analysis revealed that the deepest part of their sample dated back to roughly the 14th century. “I realized we were just short of the sediments that had been deposited during Cahokia,” Pompeani says. “We had to get a deeper core.”
In 2013, Pompeani came prepared to dig further. With an extra-long sampling tool resembling a giant metal straw, his team penetrated the lake bed down to sediment from the 7th century. The researchers were particularly interested in samples from the 12th and 13th centuries. “Sure enough there was this distinct, grayish layer, right at the time I expected,” Pompeani says. “I was like, ‘This is it.’”
Centuries ago, as fresh pollen grains, sand and clay settled into this geologic layer, a vast city bustled to the south of Horseshoe Lake. A complex of farmsteads fanned out along the Mississippi river floodplain. In the center of it all, just across the river from present-day St. Louis, vast earthen mounds towered over the metropolitan hub of the ancient Mississippian culture: Cahokia. Situated near the confluences of the Missouri, Illinois and Mississippi rivers, the city was a hub of culture and commerce. Oval hoe blades made from a rock called chert passed through Cahokia on their way to farms throughout the region while, in turn, Maize flowed into the city to feed its urban residents.
But by 1350, just 150 years after Cahokia’s peak, the city was nearly abandoned. It’s a mystery that has fascinated archaeologists, historians and, sometimes, geochemists like Pompeani. Unbeknownst to him, a second research team gathered a sediment sample from Horseshoe Lake just before his team. They published their results in a 2015 paper, which argued that the rise and fall of Cahokia was linked to fluctuations in flood frequency. At the time, news articles imagining a civilization-ending mega-flood proliferated.
When Pompeani read the paper, he balked. Though it would be six more years until he completed his analysis, now as an adjunct professor at Kansas State University, the explanation didn’t make sense to him. “A distinct geochemical layer found at the apex of Cahokia is likely to be caused by human activity — not some flood that has not been found in the archeological record,” he says.
When Pompeani completed his isotope analysis, it affirmed his suspicions. His results supported an older theory: that the decline of the Cahokia corresponded with a period of frequent drought. According to the geochemical record, the Mississippi river floodplain began to dry out during the “Little Ice Age” that cooled the earth from the 14th century until the dawn of the Industrial Revolution.
As for the sediment's distinct layer of gray, Pompeani attributes it to pollution. “If there’s any place in North America where humans had a discernible impact on the environment in prehistory, this is it,” he says. A second discolored layer is visible at the top of the sediment core. Heavy metals left over from early-1900s lead smelting operations rest on the top of Pompeani’s sample. In other words, a period of relative calm is bookended by both pre-colonial and post-industrial human impacts on the environment.
Archaeology or Allegory?
The droughts that plagued Cahokia occurred during a period of cooling global temperatures, the opposite of the earth’s current trajectory. So, as you might expect, current-day Cahokia is once again relatively wet. In a 2020 paper that assessed U.S. states' vulnerability to drought, University of Florida assistant professor of geography Johanna Engström and her co-authors at the Center for Complex Hydrosystems Research at the University of Alabama categorized Illinois as having “very low” exposure.
Despite this, Engström says that current-day Illinois could learn something from the ancient Cahokians. Like the farms that once surrounded the ancient city, the vast corn and soybean fields that spread across the state today are fed by rainfall. Unlike agricultural operations in the arid southwest, Illinois’ farms rarely install irrigation infrastructure. “The drought risk is very, very low, but what makes it vulnerable is its agriculture,” Engström says. “There is a fair bit of farmland, and not a whole lot of it is irrigated relative to other states.”
Drought does not pose an existential risk to the state’s civilization as it once did to the Missippians, but the decline of Cahokia may still be a useful allegory elsewhere in the country. Drought is likely to intensify throughout southern states in the near future. While states in the Southwest have, to some extent, adapted their infrastructure to drought conditions already, parts of the Southeast remain woefully underprepared. “Alabama, for example, has basically no water management plan,” Engström says. “You have to adapt your community and economy to the available water resources.”
If southeast states are to avoid the fate of ancient Cahokia, they will need to adapt both municipal and agricultural infrastructure to resist harm from increasing drought. This means increasing the amount of water that can be stored in reservoirs and holding ponds, building irrigation systems on farmland and streamlining water usage across the board to be more efficient and less consumptive.
“The easiest fix is to just have a water management plan,” Engström says. “Get a team of hydrologists on it and it will take maybe a year. Then you know what to do when your rivers are down 50 percent.”
Though science and technology has made modern America adaptable, our society is not immune to the consequences of a shifting climate. Just as the civilization of Cahokia was changed indelibly by deviations in temperature and precipitation, the Southeast U.S. will be changed by a warming climate. If technology and infrastructure is to save the region from crisis and decline, water managers must plan ahead