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Aerial photo of early morning over Cahokia Mounds State Historic Site.

Clues found near the site of Cahokia (above) indicated that a flood occurred at the height of the city's population and power.

Photograph by Ira Block, National Geographic

Glenn Hodges

for National Geographic

Published October 31, 2013

One thousand years ago, on a floodplain of the Mississippi River near modern-day St. Louis, the massive Native American city known today as Cahokia sprang suddenly into existence. Three hundred years later it was virtually deserted.

The reasons for Cahokia's quick emergence and precipitous decline have been among the greatest mysteries in American prehistory, but new research suggests a possible cause of the city's demise: a catastrophic flood.

A team led by Samuel E. Munoz, a doctoral student at the University of Wisconsin, Madison and National Geographic Young Explorer Grantee, reported at the 2013 conference of the Geological Society of America that their study of sediment cores from a lake adjacent to the site of Cahokia reveals calamitous flooding of the area around 1200 C.E., just as the city was reaching its apex of population and power.

While analyzing cores from Horseshoe Lake, an oxbow lake that separated from the Mississippi River some 1,700 years ago, Munoz's team discovered a layer of silty clay 19 centimeters (7.5 inches) thick deposited by a massive ancient flood.

It's unlikely that the ancient floodwaters were high enough to inundate the ten-story mound at Cahokia's center, a structure now called Monk's Mound. (See "Cahokia: America's Forgotten City.") But a flood of such magnitude would have devastated croplands and residential areas, and may have made it impossible for a population numbering as many as 15,000 to continue inhabiting the area.

Whether the flood caused Cahokia's decline and abandonment or simply contributed to it remains a subject for future research. But this much is clear: Within 150 years of the flood, what had been the largest prehistoric settlement north of Mexico became a ghost town, a vacant landscape of earthen mounds that would confound European settlers.

Though the flood is a new wrinkle in Cahokia's story, other data from the team's research supports previous archaeological conclusions about the history of Cahokia and the Mississippian culture of which it was a part.

Map of the ancient city of Cahokia.

Core Clues

Analysis of pollen deposits in the sediment cores from Horseshoe Lake shows an intensification of farming, accompanied by rapid deforestation, starting around 450 C.E., with corn cultivation peaking between 900 and 1200 C.E. Then the cores reveal the flood event, followed by a decline in corn cultivation. By 1350 C.E., the pollen record shows, agriculture there had essentially ceased.

(Related: "Drought Led to Collapse of Civilizations, Study Says.")

Munoz, a geographer who specializes in the study of pollen records, noticed that very little pollen research had been done in the American Southeast, where the Mississippian culture flourished. "And we didn't really have any studies outside big archaeological sites," he said. So when he saw Horseshoe Lake right next to Cahokia, he thought it was worth a shot.

"These floodplain lakes have been ignored for a long time as sources of these kinds of records, and they can be really valuable," said Munoz, whose research was supported by a grant from the National Geographic Society.

But he had no idea they might find such a big piece of the puzzle. "When we realized we were looking at a flood, and that it fell right at this key time in Cahokia's history, it was very exciting."

33 comments
John Clutter
John Clutter

I wonder if geologic evidence suggests a possible earthquake in the area as a cause of the flood (altering the coarse of the river) or was it a meteorological event?

Jim Ellis
Jim Ellis

As opposed to places of worship could mound building actually have been  for a functional purpose? Could they have built as something as simple as somewhere for the local inhabitants to wait out floods. Has anyone ever determined what percentage of mounds lie in flood plains.  

alan holmes
alan holmes

Hi Craig Hill,

Thanks for answering my question.

John Farrelly
John Farrelly

Having visited Cahokia and most of the largest Native American sites in the Southwest, Cahokia is every bit as spectacular. Though not made of stone as Chaco or Mesa Verde, it's size is what makes it a wonderful place to visit, but not on a hot, humid day. Great place to take your time and walk around, if only it didn't have major street right through the middle of it.

Sucka Free
Sucka Free

Every time I read an article these days, the comments section is filled with a bunch of  weirdos who have nothing better to do with themselves but beat us all over the head with their angry, annoying and otherwise trivial opinions that 99% of the time have ABSOLUTELY NOTHING to do with the article I was reading. Take all your negativity and stick it between B.C., C.E and oh yeah A.D. I think that covers all bases.

craig hill
craig hill

Had the pleasure of walking the entire complex, on a blistering day in humidity approaching that of an ocean of air. And it was worth every step. It took hours. As i walked it, it was clearly all visible. I imagined the tens of thousands who lived and visited there during great gatherings of trade, from 100s and even 1,000 miles away. Highly recommended if you share my historical-anthropological passion.

alan holmes
alan holmes

A very interesting article but could someone please explain to me what the abbreviation C.E. stands for.

Jennifer Edwards
Jennifer Edwards

The lack of historic and archaeological appreciation here is staggering. As well as the lack of understanding how studies like this are funded...

m marion
m marion

I hope a lot of time and $$ wasn't wasted on this. It is well known that that plain is subject to devastating flooding. That is why the Whiteman hasn't built any lasting settlement in the region. HEADLINE: Recent study confirms: coffee can keep you awake.

Charles Hart V
Charles Hart V

Amazing the lack of history you show there, Timothy. Some of the first modern scientists were Christians who believed in a literal 7 day creation of the universe, etc. Nothing wrong with belief (since belief ifluences everything we all do, including evolutionists and such).

sarah Bernheim
sarah Bernheim

wowwy wow with science now a days we can do and predict anything imagine how we will be in just a decade from now im in awe  speechless

Jim Miller
Jim Miller

@Jim Ellis 

Jim, 

Mound building served a dual purpose of religious significance as well as a display of status.  Important religious buildings were set upon mounds to place them closer to the Sun (the object of worship) as well as to show that the inhabitants were "above" the common rabble.  We know, or suspect this, due to the goods found in the archaeological record on and around these mounds.  

That is not to say that ALL mounds were for these purposes, but the preponderance of the evidence in the archaeological record suggests just that.

alan babb
alan babb

@John Farrelly : I always thought of Stonehenge  as being out in the middle of a great big empty  so think of my surprise when I found it right at the intersection of 2 major regional highways. Nineteenth century engineers rarely gave a hoot about ruins of any sort. I have seen the highway department tear down an Hopewell mound because some objected to kids "Parking" behind it.

craig hill
craig hill

@alan holmes Common Era. It replaces the childish voodoo fiction of Anno Domini. It reflects the fact the world accepts the Roman calendar as a common reference point, hence it's the common era of dating. The old designations didn't bring everyone in on the common dating period because it refers to the approximate and inaccurate birthdate of a specific religion's ghost.. CE does the trick.

btw BCE, Before the Common Era, has likewise replaced BC, Before Christ, for the same reason.


William Iseminger
William Iseminger

@m marion --even though there has been flooding in the past, levees have thus  far controlled most of that problem in this immediate area, so the cities of Alton, Granite City, East St. Louis, Cahokia, Dupo -- to name a few -- would certainly refute your claim there aren't any lasting settlements in the region. The village of Cahokia, about 12 miles south of Cahokia Mounds, was the first in the area, settled by the French in 1699, and it still exists. Also, this study was not to prove there were floods but to examine changes in vegetation over the centuries as reflected in the pollen grains in the sediments of the lake. Additional information was fortuitous, such as the sediment layer showing evidence of a great flood. Interestingly, at the Cahokia site itself there is not much evidence of flood deposits in house basins or pits, suggesting it was just high enough by a foot or two to miss most flooding.

William Donnelly
William Donnelly

@Charles Hart V - There's a BIG difference between "believing" in science and religious-based beliefs. And there *is* something 'wrong' with "belief", especially in contrast to truth and reality, or as much as one can muster of those based on actual facts, evidence, and proof. This is often also compounded in the negative when people use words like "know" when what they really mean is "believe" (or, "know in my heart", etc.). Just think what some of those "modern scientists" (and too many today) could have accomplished if they hadn't been blindered, constricted, and "self"-obstructed by their religious beliefs.

alan babb
alan babb

@William Iseminger @m marion : I had figured it was a fever brought by Desoto and his lot. That the river would get this high is surprising to me. Oh well. Most of the mounds and associated communities in this area (central Kentucky) are up on the bluffs with the fields in the bottoms. Must have been one bugger of a flood.

Dave Craine
Dave Craine

LOL It's us Creationist Tea Bag Tards who work to fund you're Lib (Oh my God, my brains fell out) Tard research!!!

Dave Craine
Dave Craine

@Craig Hill Time to get your head out of the sand... Obamacare is now the single biggest subsidy in the US...

Stannous Flouride
Stannous Flouride

@Dave Craine I hate to confuse you with facts but it is a FACT that the states that suck the teat of Washington the largest are the ones that elect the most Tea Party candidates.

These states take FAR MORE out of the budget than they pay in.

Gene Tylr
Gene Tylr

@craig hill @Dave Craine 

FYI ... I really liked your first post and had a great deal of empathy for you as I have also toured the site.  Now I wish I had never heard of you. 

You share neither my religion nor my general political beliefs ... which is fine. 

This is National Geographic. Not a political site.  The world is not that small and now I have an opinion of you that I frankly quite unfavorable.  Your mean spirited comments diminished my enjoyment of the article and has gained you nothing of which I am aware.  Good job.

craig hill
craig hill

@Dave Craine No you don't. You're a collective drain on the Treasury despite your taxed contributions to Nat Geo's sources. You're the single biggest parasites (assuming you count big businessmen sucking on massive titanic subsidies that are NEVER paid back) afflicitng this country.

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