What Doomed a Sprawling City Near St. Louis 1,000 Years Ago?

A thousand years ago, a city rose on the banks of the Mississippi River, near what eventually became the city of St. Louis. Sprawling over miles of rich farms, public plazas and earthen mounds, the city — known today as Cahokia — was a thriving hub of immigrants, lavish feasting and religious ceremony. At its peak in the 1100s, Cahokia housed 20,000 people, greater than contemporaneous Paris.

By 1350, Cahokia had largely been abandoned, and why people left the city is one of the greatest mysteries of North American archaeology.

Now, some scientists are arguing that one popular explanation — Cahokia had committed ecocide by destroying its environment, and thus destroyed itself — can be rejected out of hand. Recent excavations at Cahokia led by Caitlin Rankin, an archaeologist at the University of Illinois at Urbana-Champaign, show that there is no evidence at the site of human-caused erosion or flooding in the city.

Her team’s research, published in the May/June issue of Geoarcheology suggests that stories of great civilizations seemingly laid low by ecological hubris may say more about our current anxieties and assumptions than the archaeological record.

In the 1990s, interpretations of archaeological research led to the proposal that the Cahokians at the height of their city’s population had cut down many trees in the area. This practice, they said, led to widespread deforestation, erosion and increasingly severe and unpredictable local flooding.

Dr. Rankin and her colleagues set out to discover more about how Cahokia’s environment changed over the course of its development, which they hoped would test whether that hypothesis was true. Excavating in Cahokia’s North Plaza — a neighborhood in the city’s central precinct — they dug at the edge of two separate mounds and along the local creek, using preserved soil layers to reconstruct the landscape of a thousand years ago. This area had the lowest elevation, and they presumed it would have endured the worst of any flooding that had occurred.

Those soil layers showed that while flooding had occurred early in the city’s development, after the construction of the mounds, the surrounding floodplain was largely spared from major flooding until the industrial era.

“We do see some negative consequences of land clearance early on,” Dr. Rankin said, “but people deal with it somehow and keep investing their time and energy into the space.”