Correlation does not equal causation: Questioning the Great Cahokia Flood
Research Article
May 4, 2015
Munoz et al. (1) present an argument for Cahokia’s demise focusing on a massive flood event ca. A.D. 1200. Although we recognize that a flood event may have occurred, we do not agree with their hypothesis that this event (i) caused the collapse of the largest Pre-Columbian city north of Mexico and (ii) occurred ca. A.D. 1200. Cahokians did not vacate the floodplain until after the mid-14th century, preceded by new communities and social relationships during the 11th to 13th centuries. We suggest the authors reconsider their data in conjunction with the prolific evidence for community persistence and revitalization initiated during the mid-12th century.
First is the problematic nature of using wood and wood-derived charcoal for radiocarbon dating, i.e., the “old wood” problem. These samples can only provide the earliest possible date when organic material stopped absorbing atmospheric carbon and are not reliable for determining when that material was deposited in the floodplain lakes. At Horseshoe Lake, the proposed A.D. 1200 flood was preceded by a post-1200 date (UGAMS-14454, A.D. 1212–1269) and then followed by a pre-1200 date (UGAMS-13418, A.D. 992–1155). [All dates are from table S1 in Munoz et al. (1) and were calibrated using Calib 7.1* and the IntCal13 calibration.] At Grassy Lake, the flood postdates A.D. 1031 (DAMS-005576, A.D. 1031–1156) and predates a 15th century date (DAMS-0005575, A.D. 1428–1495). The A.D. 1200 date provided is a midpoint and not statistically meaningful. These dates suggest a much later depositional event that postdates Cahokia’s occupation, correlating with climatological data from the region indicating several wetter periods during the late 13th to 15th centuries (2). Furthermore, before channelization of creeks flowing into the American Bottom, Horseshoe Lake (lowest point on the floodplain) served as a floodwater reservoir of Cahokia Creek, potentially depositing silts from the uplands and surface runoff from heavy rainstorms. The hydrologic model presented by Munoz et al. is based on the modern, extensively modified, landscape. We question their date reliability and the suggested depositional processes, requesting the inclusion of nonwood radiocarbon samples with a consideration of regional archaeological data.
Second, extensive excavations in the American Bottom have revealed no evidence of large-scale flood deposits in any semisubterranean structure basins or storage pits (3). There is no archaeological evidence of a flood. In fact, excavations demonstrate large-scale construction efforts modifying the landscape to manage the movement of surface water in the Cahokia Precinct (3). Archaeological investigations throughout the American Bottom floodplain revealed the founding or reoccupation of numerous sites ca. A.D. 1200–1350, like Janey B. Goode (A.D. 1200–1275) located between the Mississippi River and Horseshoe Lake (4).
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Third, Cahokia was embedded in a network of communities that include large contemporaneous sites and a number of hamlets throughout the floodplain and surrounding uplands (4). During the period that Munoz et al. (1) pinpoint for the possible flood, archaeological evidence suggests that, instead of collapse/abandonment, a sociopolitical revitalization occurred at Cahokia including the creation of new connections to the greater southeast and continued mound construction in the central precinct (5).
References
1
SE Munoz, et al., Cahokia’s emergence and decline coincided with shifts of flood frequency on the Mississippi River. Proc Natl Acad Sci USA 112, 6319–6324 (2015).
2
LV Benson, TR Pauketat, ER Cook, Cahokia’s boom and bust in the context of climate change. Am Antiq 74, 467–483 (2009).
3
RA Dalan, et al. Envisioning Cahokia: A Landscape Perspective (Northern Illinois Univ Press, DeKalb, IL, 2003).
4
Illinois State Archaeological Survey (2008) Excavation notes (University of Illinois, Champaign–Urbana, IL).
5
JE Kelly, JA Brown, Just in time: Dating Mound 34 at Cahokia. Illinois Antiquity 45, 3–8 (2010).
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Published online: June 16, 2015
Published in issue: July 21, 2015
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The authors declare no conflict of interest.
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Correlation does not equal causation: Questioning the Great Cahokia Flood, Proc. Natl. Acad. Sci. U.S.A.
112 (29) E3753-E3753,
https://doi.org/10.1073/pnas.1509104112
(2015).
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