At its peak, between around 1050 and 1200 CE, Cahokia - the famous complex of earthen mounds about 500 kilometres southwest of Chicago, USA - wielded economic power and religious influence from the Great Lakes to the Gulf of Mexico.
     Until about 1,400 years ago, the area was prone to frequent and severe floods, with the Mississippi River rising at least 10 meters. Beginning around 600 CE big floods became less frequent. Indigenous peoples moved into the floodplain and began to farm more intensively.
     Around 900 CE, people in the area began to cultivate maize and their population exploded, shown by the number and size of buildings and structures that sprang up in the region. By the mid-11th century these settlements had grown into a metropolis with a population of at least 10,000 in its central district.
     Starting around 1200 CE, the climate became wetter again, and large floods returned with increasing frequency. Sediment cores show evidence of at least eight major flood events in the central Mississippi River valley. Layers above and below flood sediments allowed researchers to date the events.
     Dr Sissel Schroeder, a Wisconsin archaeologist who collaborated in the research, says the return of the floods coincides closely with many signs of political instability and social upheaval: "We see some important changes in the archaeology of the site at this time, including a wooden wall that is built around the central precinct of Cahokia. There are shifts in craft production, house size and shape, and other signals in material production that indicate political, social, and economic changes that may be associated with social unrest."
     Sam Munoz, a geographer at the University of Wisconsin and lead author of the study, was originally looking for signs of prehistoric land use on ancient forests. He chose Cahokia because it was such a large site. At one point, tens of thousands of people lived in and around Cahokia. If there was anywhere that ancient peoples would have altered the landscape, it was around Cahokia.
     Munoz adds that: "Beyond the Cahokia site, our results demonstrate how sensitive large rivers like the Mississippi are to climatic variability - and how dependent human societies are on rivers."

Edited from Popular Archaeology, Western Digs (4 May 2015)