Published: January 2011 Cahokia 
 
 By Glenn Hodges 
 
 Photographs by Don Burmeister and Ira Block 
 
 If they ever build a Wal-Mart at Machu Picchu, I will think of Collinsville Road. 
 
 I'm standing at the center of what was once the greatest civilization between the deserts of Mexico and the North American Arctic—America's first city and arguably American Indians' finest achievement—and I just can't get past the four-lane gash that cuts through this historic site. Instead of imagining the thousands of people who once teemed on the grand plaza here, I keep returning to the fact that Cahokia Mounds in Illinois is one of only eight cultural World Heritage sites in the United States, and it's got a billboard for Joe's Carpet King smack in the middle of it. 
 
 But I suppose Cahokia is lucky. Less than ten miles to the west, the ancient Indian mounds that gave St. Louis the nickname Mound City in the 1800s were almost completely leveled by the turn of the century. Today only one survives, along with some photographs and a little dogleg road named Mound Street. The relentless development of the 20th century took its own toll on Cahokia: Horseradish farmers razed its second biggest mound for fill in 1931, and the site has variously been home to a gambling hall, a housing subdivision, an airfield, and (adding insult to injury) a pornographic drive-in. But most of its central features survived, and nearly all of those survivors are now protected. Cahokia Mounds may not be aesthetically pristine, but at 4,000 acres (2,200 of which are preserved as a state historic site), it is the largest archaeological site in the United States, and it has changed our picture of what Indian life was like on this continent before Europeans arrived. 

 by Kelly April Tyrrell 
 
 A painting of Cahokia Mounds State Historic site by William R. Iseminger. Horseshoe Lake, where core sediments give a timeline of flooding from the Mississippi River, can be seen in the upper left. Courtesy of William R. Iseminger 
 
 As with rivers, civilizations across the world rise and fall. Sometimes, the rise and fall of rivers has something to do with it. 
 
 At Cahokia, the largest prehistoric settlement in the Americas north of Mexico, new evidence suggests that major flood events in the Mississippi River valley are tied to the cultural center’s emergence and ultimately, to its decline. 
 
 Publishing May 4 in the Proceedings of the National Academy of Sciences, a research team led by UW-Madison geographers Samuel Munoz and Jack Williams provides this evidence, hidden beneath two lakes in the Mississippi floodplain. Sediment cores from these lakes, dating back nearly 2,000 years, provide evidence of at least eight major flood events in the central Mississippi River valley that could help explain the enigmatic rise and fall of Cahokia, near present-day St. Louis. 
 
 While the region saw frequent flood events before A.D. 600 and after A.D. 1200, Cahokia rose to prominence during a relatively arid and flood-free period and flourished in the years before a major flood in 1200, the study reveals. That was also a time of political instability and population decline. Two hundred years later, Cahokia was completely abandoned. 
 
 A modeled map of Cahokia and present-day St. Louis after the historic 1844 flood of the Mississippi River. Courtesy of Samuel Munoz View larger 
 
 While drought has traditionally been implicated as one of several factors leading to the decline of many early agricultural societies in North America and around the world, the findings of this study present new ideas and avenues for archaeologists and anthropologists to explore. 
 
 “We are not arguing against the role of drought in Cahokia’s decline but this presents another piece of information,” says Munoz, a Ph.D. candidate in geography and the study’s lead author. 
 
 “It also provides new information about the flood history of the Mississippi River, which may be useful to agencies and townships interested in reducing the exposure of current landowners and townships to flood risk,” says Williams, a professor of geography and director of the Nelson Institute for Environmental Studies Center for Climatic Research. 
 
 Munoz didn’t set out intending to study flooding. His findings were “kind of an accident,” he says. 
 
 Originally, Munoz was looking for the signals of prehistoric land use on ancient forests. He chose to study Cahokia because it was such a large site and is famous for its large earthen mounds. At one point, tens of thousands of people lived in and around Cahokia. If there was anywhere that ancient peoples would have altered the landscapes of the past, it was Cahokia. 
 
 The team went to Horseshoe Lake, near the six-square-mile city’s center, and collected cores of lake mud — all the stuff that settles to the bottom — to look for pollen and other fossils that document environmental change. Lakes are “sediment traps” that can capture and record past environmental changes, much like the rings of a tree. 
 
 At Cahokia, Monk’s Mound, the largest earthwork built north of Mexico prior to the arrival of Europeans. Photo: Sissel Schroeder 
 
 “We had these really strange layers in the core that didn’t have any pollen and they had a really odd texture,” Munoz says. “In fact, one of the students working with us called it ‘lake butter.’” 
 
 They asked around, talked to colleagues, and checked the published literature. The late Jim Knox, who spent his 43-year career as a geography professor at UW-Madison, suggested to Munoz that he think about flooding, which can disrupt the normal deposition of material on lake bottoms and leave a distinct signature. 
 
 The team used radiocarbon dating of plant remains and charcoal within the core to create a timeline extending back nearly two millennia. In so doing, they established a record of eight major flood events at Horseshoe Lake during this time, including the fingerprint left by a known major flood in 1844. 
 
 To validate the findings, the team also collected sediments from Grassy Lake, roughly 120 miles downstream from Cahokia, and found the same flood signatures (Grassy Lake is younger than Horseshoe Lake, so its sediments captured only the five most recent flood events). 
 
 The new findings show that floods were common in the region between A.D. 300 and 600. Meanwhile, the earliest evidence of more agricultural settlement appears along the higher elevation slopes at the edge of the central Mississippi River floodplain around the year 400. But by 600, when flooding diminished and the climate became more arid, archaeological evidence shows that people had moved down into the floodplain and begun to increase in population and farm more intensively. 
 
 Samuel Munoz 
 
 “Rarely do you get such fortuitous opportunities where you have these nice sedimentary records next to an archaeological site that’s so well studied,” says Munoz. 
 
 Early on in the study, Munoz and Williams enlisted the help of Sissel Schroeder, a UW-Madison professor of anthropology whose doctoral studies focused on the Cahokia area. Schroeder accompanied the Geography Department scientists out in the field and helped provide historical and archaeological context. 
 
 She explains that while there has been little archaeological evidence to suggest flooding at Cahokia, it can’t be ruled out. It’s possible, she says, that researchers have simply missed the signals. 
 
 For example, archaeologists know that around the year 900, 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. Archaeologists often think of Cahokia as a chiefdom, with a hierarchy of smaller settlements that spread out from the city, much like the small county seats that surround the major government centers we’re familiar with today, Schroeder explains. 
 
 But around 1200, coinciding with a major flood fingerprint in Munoz’s sediments, the population began to decline along with other shifts in the archaeological record. 
 
 “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,” says Schroeder. “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.” 
 
 Sissel Schroeder 
 
 Cahokia appears to have fractured and its people began to migrate to other parts of North America. By 1400, after the arid conditions that suppressed large floods and favored Cahokia’s rise had passed, it was deserted. 
 
 While many factors likely contributed to Cahokia’s decline — from extreme events like droughts or floods, to the inherent instability archaeologists and anthropologists have documented in other chiefdom societies — a major flooding event could have been the proverbial last straw. 
 
 “It would have had a particularly destabilizing effect after hundreds of years without large floods,” Schroeder says. 
 
 In order to deposit sediments into Horseshoe and Grassy Lakes, the Mississippi River would have had to rise 10 meters (about 33 feet) above its base elevation at St. Louis, according to models run in the study. This substantial flood would have inundated the region’s crops, impacted essential food stores, and created agricultural shortfalls. 
 
 Food and other essential resources would have been currency in a civilization like Cahokia and could have been leveraged for political gains following a flood of the scale documented in the study. 
 
 “We hope archaeologists can start integrating these flood records into their ideas of what happened at Cahokia and check for evidence of flooding,” says Munoz, who plans to continue studying flood records in lakes around the country once he graduates this year. 
 
 Jack Williams 
 
 The study also provides new information about the river’s behavior in the central Mississippi Valley, Williams says. Relatively little is currently known about its prehistoric flood cycle but the study suggests that major floods like those in 1844 or 1993 happened every century or two prior to European settlement and intervention, with the exception of the unusually arid years that facilitated Cahokia’s growth. 
 
 “We have managed the river so much to prevent floods from happening, we don’t have a good baseline for how the river behaves without human modification,” he says. “This may help us understand not only how it once behaved, but how it may behave in the future.” 
 
 The study was supported by the National Science Foundation, the National Geographic Society, the National Lacustrine Core Facility, The Geological Society of America, and through a Packard Foundation fellowship to study co-author David Fike at Washington University in St. Louis. Kristine Gruley and Ashtin Massie, both at UW-Madison, also co-authored the study. 

 View Images Once the largest prehistoric settlement north of Mexico, Cahokia became a ghost town whose earthen mounds mystified European settlers. 
 
 Photograph by Ira Block, National Geographic Creative 
 
 Researchers have long debated the reasons behind the rapid rise and swift disappearance of Cahokia , a sprawling, ancient city-state near the modern city of St. Louis. Now an analysis of sediment cores reveals that the city’s ups and downs correspond to the timing of Mississippi River megafloods, according to a recent study . 
 
 Archaeological data show that agricultural settlements first appeared in the area around A.D. 400. Around A.D. 1050 there was a veritable boom at Cahokia, which became a major political and cultural center with a population in the tens of thousands. But by 1350—a span of only three centuries—Cahokia was gone. ( Discover Cahokia .) 
 
 To uncover clues to the city’s fate, a research team led by University of Wisconsin-Madison geographers Samuel Munoz and Jack Williams performed laser diffraction particle size analysis on sediment samples from Horseshoe Lake, an oxbow lake near Cahokia. The samples yielded evidence of eight separate flood events over the past 2,000 years. 
 
 America'sforgotten city At its peak in the mid-12th century, Cahokia was by far the largest native community in North America. Smaller settlements shared similar features, including platform mounds, large plazas, and protective stockades. Mississippian culture Area of influence Town or mound center MINN. WIS. IOWA ILL. UNITED STATES IND. MO. Cahokia VA. KY. Mississippi River N.C. TENN. OKLA. ARK. S.C. GA. MISS. ALA. TEX. LA. Gulf of Mexico FLA. 200 mi 200 km Cahokia Creek Woodhenge North Plaza Canteen Creek Monks Mound Grand Plaza Stockade Borrow pit Area where earth was extracted to build the mounds Mound 72 Distance from Monks Mound to Mound 72 is about 0.5 miles (0.8 kilometers). JEROME N. COOKSON, NG STAFFArt: Greg HarlinSources: Bill Iseminger and Mark Esarey, Cahokia Mounds State Historic Site; John Kelly, Washington University in St. Louis 
 
 The researchers discovered that changes in the magnitude and frequency of the newly identified floods correspond with archaeological evidence for changes in population, land use, and settlement over the full history of Cahokia. 
 
 A flood-free period begins around A.D. 600, a time when settlements moved from higher slopes along the edges of the Mississippi floodplain down to the floodplain proper. Crops were cultivated more intensely and the population began to grow. In time Cahokia would become the largest prehistoric settlement in the United States . 
 
 So what caused the city’s downfall? Drought, overexploitation of resources, and human conflict have all been proposed as reasons behind the end of Cahokia. But an earlier study of sediments from Horseshoe Lake suggested that major flooding had occurred in the area around 1200. 
 
 Munoz and his team analyzed sediments from another oxbow lake 120 miles (190 km) downstream of Horseshoe Lake and found confirming evidence of the catastrophic flood. The Mississippi River rose more than 33 feet (10 meters) and played a critical role in the total abandonment of Cahokia within 150 years. 
 
 Munoz, whose research was funded in part by a Young Explorers Grant from the National Geographic Society, hopes the findings encourage archaeologists to add prehistoric flood records, when available, to their toolkit. “It’s always great when we can integrate geoarchaeology into traditional archaeology,” he says.
– For Native Americans living in the 12th century, Cahokia was the place to be. Now researchers think they've uncovered a major reason why the hub located near what is now St. Louis disappeared—massive flooding of the Mississippi River. Researchers dug up soil samples from two local lakes and found telltale evidence of catastrophic flooding around the time that Cahokia was flourishing. “We’re talking big floods, once a century,” a researcher from the University of Wisconsin-Madison tells the St. Louis Post-Dispatch. It seems that the region enjoyed an unusually flood-free period from roughly 600AD to 1200AD. In that time, people flocked to the fertile region, turning Cahokia into what National Geographic calls the "largest prehistoric settlement in the United States." (Another National Geographic piece frames it as "America's first city.") But they may have gotten too complacent. People began moving from higher elevations into what we now know to be a flood plain, and the population boomed with the cultivation of maize. It peaked around the mid-12th century, but the resumption of regular floods appeared to begin taking a toll. By about 1350, the city had been abandoned. Previous theories on its demise have focused on deforestation, overhunting, political volatility, and, especially, drought. “We are not arguing against the role of drought in Cahokia’s decline, but this presents another piece of information,” says the lead author of the study at the university. Another researcher suggests the flooding could have been the final straw. “It would have had a particularly destabilizing effect after hundreds of years without large floods." (Maybe an ancient brewery would have helped?)