Long before Europeans arrived to settle St. Louis, an impressive human construction stood on the eastern side of the Mississippi River. It was the Native American city of Cahokia. At its height, tens of thousands lived in and around Cahokia, leaving behind great earthen mounds as testament. The largest still stands about a hundred feet tall today, minus what was likely a temple that once adorned its crest.

Like all societies that disappear, we naturally wonder what brought this one to an end around 1350 AD, after a run of hundreds of years. Several familiar scenarios have been proposed: drought, over-exploitation of natural resources, and conflict. However, rather than the onset of a drought, it may have been the end of a dry period that did in Cahokia.

Cahokia was built near the Mississippi River and within its floodplain, and it wasn't protected by any of the levees and flood control structures that exist today. Samuel Munoz of the University of Wisconsin-Madison and a group of collaborators set out to find a record of floods on the Mighty Mississippi that might have impacted the denizens of Cahokia. Some previous work with a sediment core from a nearby lake contained what appeared to be a flood-deposited layer from around 1200 AD—the start of Cahokia’s precipitous decline. To find out more, they collected a second core from another floodplain lake about 200 kilometers downstream and focused on reconstructing the flood history in detail.

The sediment at the bottom of these lakes was mostly dark brown silt, but this was broken up by layers of gray or tan silty clay largely devoid of organic material—the same sort of sediment that settles out in these lakes today after the Mississippi floods its banks. The researchers measured the distribution of sediment grain sizes along these two lake cores, identifying the sections with a higher proportion of clay. From that, they were able to pick out major flood events and assign dates based on carbon-dated bits of wood from the cores.

One core goes back to about 900 AD, but the other extends all the way to 200 AD. Together, they show a notable coincidence: there were no significant floods between about 600 AD and 1200 AD, which happens to have been Cahokia’s time in the sun. Large floods occurred with some frequency in the earlier and later portions of the record, including the prominent one around 1200 AD.

That gap isn’t just a chance string of uneventful years. It was a period of low precipitation across the region. In the Southwest, for example, a decades-long drought (the worst of the millennium) hit in the 1100s, coinciding with the Medieval Warm Period of the North Atlantic.

It seems that a flooding hiatus allowed settlement and agriculture in the lowlands of the floodplain to flourish. That prosperity might have come to an end when the hiatus did, and came as part of a large collection of significant changes. The researchers write, “Extensive inundation of the floodplain was unprecedented for the sociopolitical system established at Cahokia, and the return of large floods at ca. A.D. 1200 at the onset of regional depopulation, agricultural contraction, political decentralization, the construction of defensive palisades, destruction of outlying population centers, and decline of monumental construction at Cahokia implicate flooding as a factor in the reorganization of Cahokia’s sociopolitical structure that initiated its decline.”

Societies are complex, and it’s always dangerous to latch on to simple narratives when the fog of history obscures many details. But it makes sense that this catastrophe could have been a turning point. After all, Cahokia itself withered to nothing over the century-and-a-half that followed.

Just as decisions about water availability in the American Southwest were made during the unusually wet early-1900s, the people of Cahokia didn’t realize they were enjoying conditions unrepresentative of the longterm average. That, as much as anything, may have doomed the city’s longterm survival—though six hundred years ain’t bad.

PNAS, 2015. DOI: 10.1073/pnas.1501904112 (About DOIs).