North plaza in Cahokia was likely inundated year-round

The ancient North American city of Cahokia had as its focal point a feature now known as Monks Mound, a giant earthwork surrounded on its north, south, east and west by large rectangular open areas. These flat zones, called plazas by archaeologists since the early 1960s, were thought to serve as communal areas that served the many mounds and structures of the city.

New paleoenvironmental analyses of the north plaza suggest it was almost always underwater, calling into question earlier interpretations of the north plaza’s role in Cahokian society. The study is reported in the journal World Archaeology.

Cahokia was built in the vicinity of present-day St. Louis, beginning in about A.D. 1050. It grew, thrived for more than 300 years and was abandoned by 1400. Many mysteries surround the culture, layout and architecture of the city, in particular its relationship to water. Cahokia was built in a flood plain below the confluence of the Mississippi and Illinois rivers and would have been regularly infiltrated with flowing water, said Caitlin Rankin, a geoarchaeologist at the Illinois State Archaeological Survey who conducted the new research.

“Cahokia is the largest archaeological site in North America, but only about 1% of it has been excavated, so there’s so much about the site that we don’t know,” Rankin said.

Early in her encounters with the city’s layout, Rankin was baffled by the location and height of the north plaza.

“It’s a really strange area because it’s at a very low elevation, like the lowest elevation of the site,” she said. “And it’s in an old meander scar of the Mississippi River.”

Two creeks ran through the area, and it likely flooded whenever the Mississippi swelled after heavy rains.

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