As the river rises: Cahokia's emergence and decline linked to Mississippi River flooding

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 today [5/4/15] 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. Cahokia, in the midst of political instability and population decline at this time, was completely abandoned by the year 1400.

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 Samuel 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.

However, Munoz never intended to make these findings. In fact, "it was 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 in the area around 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.

"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, began to increase in population, and farmed more intensively.

"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."

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.

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."
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Reference:

EurekAlert2015. “As the river rises: Cahokia's emergence and decline linked to Mississippi River flooding”. EurekAlert. Posted: May 4, 2015. Available online: http://www.eurekalert.org/pub_releases/2015-05/uow-atr043015.php

Scientists Find Evidence of Ancient Megadrought in Southwestern U.S.

A new study at the University of Arizona's Laboratory of Tree-Ring Research has revealed a previously unknown multi-decade drought period in the second century A.D. The findings give evidence that extended periods of aridity have occurred at intervals throughout our past.

Almost 900 years ago, in the mid-12th century, the southwestern U.S. was in the middle of a multi-decade megadrought. It was the most recent extended period of severe drought known for this region. But it was not the first.

The second century A.D. saw an extended dry period of more than 100 years characterized by a multi-decade drought lasting nearly 50 years, says a new study from scientists at the University of Arizona.

UA geoscientists Cody Routson, Connie Woodhouse and Jonathan Overpeck conducted a study of the southern San Juan Mountains in south-central Colorado. The region serves as a primary drainage site for the Rio Grande and San Juan rivers.

"These mountains are very important for both the San Juan River and the Rio Grande River," said Routson, a doctoral candidate in the environmental studies laboratory of the UA's department of geosciences and the primary author of the study, which is upcoming in Geophysical Research Letters.

The San Juan River is a tributary for the Colorado River, meaning any climate changes that affect the San Juan drainage also likely would affect the Colorado River and its watershed. Said Routson: "We wanted to develop as long a record as possible for that region."

Dendrochronology is a precise science of using annual growth rings of trees to understand climate in the past. Because trees add a normally clearly defined growth ring around their trunk each year, counting the rings backwards from a tree's bark allows scientists to determine not only the age of the tree, but which years were good for growth and which years were more difficult.

"If it's a wet year, they grow a wide ring, and if it's a dry year, they grow a narrow ring," said Routson. "If you average that pattern across trees in a region you can develop a chronology that shows what years were drier or wetter for that particular region."

Darker wood, referred to as latewood because it develops in the latter part of the year at the end of the growing season, forms a usually distinct boundary between one ring and the next. The latewood is darker because growth at the end of the growing season has slowed and the cells are more compact.

To develop their chronology, the researchers looked for indications of climate in the past in the growth rings of the oldest trees in the southern San Juan region. "We drove around and looked for old trees," said Routson.

Literally nothing is older than a bristlecone pine tree: The oldest and longest-living species on the planet, these pine trees normally are found clinging to bare rocky landscapes of alpine or near-alpine mountain slopes. The trees, the oldest of which are more than 4,000 years old, are capable of withstanding extreme drought conditions.

"We did a lot of hiking and found a couple of sites of bristlecone pines, and one in particular that we honed in on," said Routson.

To sample the trees without damaging them, the dendrochronologists used a tool like a metal screw that bores a tiny hole in the trunk of the tree and allows them to extract a sample, called a core. "We take a piece of wood about the size and shape of a pencil from the tree," explained Routson.

"We also sampled dead wood that was lying about the land. We took our samples back to the lab where we used a visual, graphic technique to match where the annual growth patterns of the living trees overlap with the patterns in the dead wood. Once we have the pattern matched we measure the rings and average these values to generate a site chronology."

"In our chronology for the south San Juan mountains we created a record that extends back 2,200 years," said Routson. "It was pretty profound that we were able to get back that far."

The chronology extends many years earlier than the medieval period, during which two major drought events in that region already were known from previous chronologies.

"The medieval period extends roughly from 800 to 1300 A.D.," said Routson. "During that period there was a lot of evidence from previous studies for increased aridity, in particular two major droughts: one in the middle of the 12th century, and one at the end of the 13th century."

"Very few records are long enough to assess the global conditions associated with these two periods of Southwestern aridity," said Routson. "And the available records have uncertainties."

But the chronology from the San Juan bristlecone pines showed something completely new:

"There was another period of increased aridity even earlier," said Routson. "This new record shows that in addition to known droughts from the medieval period, there is also evidence for an earlier megadrought during the second century A.D."

"What we can see from our record is that it was a period of basically 50 consecutive years of below-average growth," said Routson. "And that's within a much broader period that extends from around 124 A.D. to 210 A.D. -- about a 100-year-long period of dry conditions."

"We're showing that there are multiple extreme drought events that happened during our past in this region," said Routson. "These megadroughts lasted for decades, which is much longer than our current drought. And the climatic events behind these previous dry periods are really similar to what we're experiencing today."

The prolonged drought in the 12th century and the newly discovered event in the second century A.D. may both have been influenced by warmer-than-average Northern Hemisphere temperatures, Routson said: "The limited records indicate there may have been similar La Nina-like background conditions in the tropical Pacific Ocean, which are known to influence modern drought, during the two periods."

Although natural climate variation has led to extended dry periods in the southwestern U.S. in the past, there is reason to believe that human-driven climate change will increase the frequency of extreme droughts in the future, said Routson. In other words, we should expect similar multi-decade droughts in a future predicted to be even warmer than the past.

Routson's research is funded by fellowships from the National Science Foundation, the Science Foundation Arizona and the Climate Assessment of the Southwest. His advisors, Woodhouse of the School of Geography and Development and Overpeck of the department of geosciences and co-director of the UA's Institute of the Environment, are co-authors of the study.
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References:

Science Daily. 2011. "Scientists Find Evidence of Ancient Megadrought in Southwestern U.S.". Science Daily. Posted: November 6, 2011. Available online: http://www.sciencedaily.com/releases/2011/11/111106151505.htm

600-year-old artifact found in Hells Canyon

A hiker stumbled upon a really old piece of Idaho history in Hells Canyon. Now, archaeologists know just how old it is. It dates back centuries.

"We know that people have lived in Idaho for at least 130 centuries," said State Archaeologist Dr. Ken Reid.

And those people left evidence of their lives. Their artwork in the form of petroglyphs and pictographs decorates the rocks and cliffs in Hells Canyon. Their house pits sit in neighborhoods along the banks of the Snake River.

Hells Canyon is beautiful. It's also rich in history.

"There's an intact outdoor museum really of Idaho's past that survives," said Dr. Reid.

A hiker found part of that surviving past under a rock pile under a rock ledge made by a huge boulder.

"It was a perfect place to get some shade on a hot hike down the Snake River Trail," Reid said.

In March of 2008, the hiker found a cache of Nez Perce textiles made of cedar bark. Idaho State Archaeologist Dr. Ken Reid and a team of researchers excavated the site. Reid believes the materials would have been woven into a mat or basket. His theory is that a Nez Perce woman placed the bundle under the stones for safekeeping.

"Presumably the woman who did it expected to come back and retrieve it at some point in the future," Reid said. "And for whatever reason, did not."

The researchers did retrieve it and sent it off to find out how old it is. After a couple more years of fundraising, finding volunteer help and testing, they recently got their answer.

"The basket was left there some time between A.D. 1395 and 1435," said Dr. Reid.

Dr. Reid says two carbon dates put the bundle at nearly 600 years old. In other words, the Nez Perce woman placed it there at least 60 years before Columbus sailed the ocean blue.

Dr. Reid calls the discovery unprecedented. He said no textiles of any kind had ever been found in Hells Canyon before this.

"It was exciting," said Dr. Reid. "You don't find things like this often enough."

Researchers at Washington State University are studying the textiles now. Dr. Reid ultimately hopes the materials will be displayed at the Nez Perce Museum in Spalding in Northern Idaho, which he says has facilities to preserve the perishable, fragile fragment of the past. The bundle belongs to the Forest Service, but he believes it belongs in Idaho.

Along with the bundle of textiles, Dr. Reid says they also found what they believe is a wooden weaving tool and a foot long stone pestle that would have been used to crush roots or other food.
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References:

Petcash, Doug. 2011. "600-year-old artifact found in Hells Canyon". NWCN. Posted: July 22, 2011. Available online: http://www.nwcn.com/home/?fId=125954468&fPath=/news/local&fDomain=10227