Ancient Native American city may have been done in by Mississippi floods

Posted: May 7, 2018 by oldbrew in climate, History, Natural Variation

Monks Mound, part of Cahokia

The theory here is that the Cahokians had no experience of Mississippi floods for several centuries, likely due to the drought-prone Medieval Warm Period, and had no defence when one eventually did occur.
As one expert (Pauketat) noted:
“Cahokia was so large – covering three to five square miles – that archaeologists have yet to probe many portions of it. Its centerpiece was an open 50-acre Grand Plaza, surrounded by packed-clay pyramids. The size of 35 football fields, the Grand Plaza was at the time the biggest public space ever conceived and executed north of Mexico.”

In those days of course there were no oil or power companies for agitators to point the finger of blame at for such natural disasters, and they didn’t know they were on a flood plain.

Cahokia’s run just happens to line up with a hiatus for the area, as Scott Johnson explained in this 2015 article.

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.

Continued here.

Study: PNAS, 2015. DOI: 10.1073/pnas.1501904112

See also:
Possible impacts of early-11th-, middle-12th-, and late-13th-century droughts on western Native Americans and the Mississippian Cahokians [2006]

One or more of three intense and persistent droughts impacted some Native American cultures in the early-11th, middle-12th and late-13th centuries, including the Anasazi, Fremont, Lovelock, and Mississippian (Cahokian) prehistorical cultures. Tree-ring-based reconstructions of precipitation and temperature indicate that warm drought periods occurred between AD 990 and 1060, AD 1135 and 1170, and AD 1276 and 1297. These droughts occurred during minima in the Pacific Decadal Oscillation and may have been associated with positive values of the Atlantic Multidecadal Oscillation.
[continues here]

  1. oldbrew says:

    The abstract of another study says:

    Alluvial valley fills from tributaries to the upper Republican River, southwest Nebraska, USA, provide soil- and morpho-stratigraphic evidence for an episode of channel incision that occurred between c. 1100 and 800 14C yr BP, based on 11 new radiocarbon ages. This local episode of channel incision correlates with other alluvial stratigraphic studies from the central Great Plains and demonstrates regionally synchronous fluvial behaviour. Proxy records of palaeohydrologic conditions in and around the Great Plains (including lacustrine, aeolian and geomorphological sources) indicate that channel incision correlates with a multicentennial episode of common, widespread drought. Temporally, this drought episode corresponds to the period recognized in many regions as the Mediaeval Warm Period (MWP). Therefore, this research demonstrates: (1) a relationship in the semi-arid central Great Plains between drought and increased fluvial incision; and (2) a local/regional geomorphic response to a particular episode of global climate change. [bold added]

    Alluvial stratigraphic evidence for channel incision during the Mediaeval Warm Period on the central Great Plains, USA [2005]

  2. crosspatch says:

    California had a 200 year long period of drought that ended at about that time and they had another, shorter “megadrought” a little later.

    [reply] interesting report

  3. oldbrew says:

    Study: Ancient mound builders carefully timed their occupation of coastal Louisiana site

    That so many mound sites have survived in coastal Louisiana is a testament to their careful construction, Mehta said. Neglect, however, and coastal subsidence – the result of engineered changes to the flow of the Mississippi River – are wearing away at the mounds.

    “Louisiana loses about two ancient mounds and/or Native American villages a year,” Mehta said.

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