North 'plaza' in Cahokia was likely inundated year-round, study finds

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 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.

Researchers took samples from nearby wetlands to determine the environmental history of the north plaza in Cahokia. Credit: Caitlin Rankin

The study focused on the north plaza, an expanse at a low elevation that is almost always inundated with water. Credit: Caitlin Rankin

Sediments from excavations at Mound 5 reveal that the north plaza was a wetland prior to, and after, mound construction. Credit: Caitlin Rankin

Caitlin Rankin stands in a trench dug in Mound 16, one of four mounds that delineate the north plaza. Credit: Ann Merkle

Rankin collects baseline data from a nearby prairie. Credit: Grace Ward