The people who built and lived among the tall, sculpted mounds now preserved at Cahokia Mounds Historic Site have long presented a mystery to archeologists.

One of the biggest mysteries: Why did they leave?

A team of geographers studying pollen deposits buried in the sediment under Horseshoe Lake may have stumbled upon new evidence that helps explain Cahokia’s decline.

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The answers are in the lake butter

With boats and an array of sampling equipment, researchers from the University of Wisconsin pulled up long tubes of sediment cores from beneath the lake. Lakes naturally collect all sorts of environmental data: grains of pollen, small plants or animals and other micro-fossils naturally settle to the lake’s bottom and create a record of history.

“It looks like a two-to-three-inch diameter tube of mud,” said Ph.D. candidate Sam Munoz. “Each meter represents about 500 years of time.”

The cores from Horseshoe Lake revealed several unusually smooth-textured sediment layers that were lighter in color than the rest. One student coined the term “lake butter” to describe it.

“These were actually layers that were deposited by floods. When the Mississippi River flooded and flooded high enough, it would deposit all the mud it was carrying in our lake,” Munoz said.

After a particle size analysis, the team concluded that the layers were the result of a series of major flood events on the Mississippi River. They corroborated the data by collecting sediment cores and dating potential flood events at Grassy Lake, about 100 miles down river.

The lakes showed evidence of five major floods over the past 1000 years. It’s hard to pinpoint exactly when the floods occurred, but the geologists narrowed the dates down to five approximate years, two of which correspond to major population declines at Cahokia:

Sam Munoz | University of Wisconsin

1100-1260 A.D. The year 1200 is the most commonly cited date for when Cahokia’s population began to decline. The first stockade was built around 1175, suggesting Cahokia’s residents were concerned about potential attacks.

1340-1460 A.D. By the middle of the 1300’s, archeologists believe Cahokia was largely abandoned. A mega-drought in the southwestern United States occurred around 1375, but it’s unclear if it reached Cahokia.

The geographers ran simple models to estimate just how big a flood would have to be in order to show up in the sediment cores of Horseshoe Lake.

“In order for us to see these floods in our lake, the river needs to get about 10 meters (33 feet) above its base elevation above St. Louis,” Munoz said. “That’s a big flood.”

Munoz and his team published their findings in Proceedings of the National Academy of Sciences.

Pinpointing Cahokia’s decline

Today, the mounds at Cahokia Mounds State Historic Site are covered with thick, green grass and a few wildflowers. Groups of schoolchildren and tourists hike around the footpaths, and traffic roars along Collinsville Road, which cuts right between the largest mound and the field that was once the main plaza.

Cahokia rose to prominence as an economic and spiritual center for the Mississippian people around 1050 A.D. Archeologists estimate its population reached 20,000 people—the largest city north of Mexico at the time. But around 1200, Cahokia’s population began to decline. One-hundred-fifty years later, the thriving metropolis was almost entirely abandoned.

Durrie Bouscaren | St. Louis Public Radio

“So the question is what happens to the people? Why does this happen? Now that we have the flood, is that what’s causing what we’re seeing or is it just part of a bigger picture that’s taking place?” said Washington University archeologist John Kelly.

Even though Cahokia is 6.3 miles east of the banks of the Mississippi River, it sits in the floodplain. The city would have been inundated.

“You would have had floodwaters coming up about a third of the way up the mound here,” Kelly said, speaking from the top of Monk's Mound, the highest mound at the site. As the waters from Horseshoe Lake continued to rise, people would have left, he said.

Cahokia’s population boom between 1050 and 1200 occurred during an extensive dry period in the region, which may have made weather patterns more predictable. Cultivation of native plants such as sunflowers, corn and goosefoot (a grain similar to quinoa) flourished. But the flood that occurred around 1200 likely marked the end of that arid period.

“With these floods, once that water recedes they can come back and reclaim it,” Kelly said. “But can they plant the next year’s crop?”

Durrie Bouscaren | St. Louis Public Radio

A 2010 book by Bill Iseminger, assistant site manager at Cahokia, summarized a number of theories for the region’s population decline. For example, the city’s growing population became unsustainable, leading to resource depletion and deforestation. A “Little Ice Age” experienced by much of North America in the 1200’s may have affected crops and made weather patterns hard to predict. Internal turmoil such as political instability, warfare and the decline of trade may have led some residents to seek new places to live. Archeologists continue to debate different theories, and the truth likely lies in a combination of factors.

“Did the flood cause a sequence of events to take place here, or was the flood happening at the same time as these other events and it just exacerbated the problem?” Kelly asked.

He said additional investigations into silt deposits around the site can help pinpoint the exact dates of the floods, and what impact they may have made. If climate changes pushed the residents of Cahokia away, maybe modern day residents of the St. Louis region should take note.

“Unfortunately, with our politicians and developers, you can’t get across to them how powerful that Mississippi River is,” Kelly said. “You don’t see places like [Cahokia] sitting on the Mississippi River.”

For more news on health, science and the environment, follow Bouscaren on Twitter: @durrieB

An earlier version of this article incorrectly identified the team from the University of Wisconsin as geologists. They are geographers.