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USGS Staff -- Published Research US Geological Survey

2009

Cahokia's Boom and Bust in the Context of Climate Change

Larry Benson U.S. Geological Survey, [email protected]

Timothy R. Pauketat University of Illinois at Urbana-Champaign, [email protected]

Edward R. Cook Lamont-Doherty Earth Observatory, [email protected]

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Benson, Larry; Pauketat, Timothy R.; and Cook, Edward R., "Cahokia's Boom and Bust in the Context of Climate Change" (2009). USGS Staff -- Published Research. 724. https://digitalcommons.unl.edu/usgsstaffpub/724

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. CAHOKIA'S BOOM AND BUST IN THE CONTEXT OF CLIMATE CHANGE

Larry V. Benson, Timothy R. Pauketat, and Edward R. Cook

During the early Mississippian Lohmann phase (A.D. 1050-1100), theAmerican Bottom experienced a political and eco nomic transformation. This transformation included the abrupt planned construction of central Cahokia, a large-scale influx of people to "downtown Cahokia," the abandonment of pre-Mississippian village settlements, the reorganization offarm ing in theMississippi River floodplain, and thefounding of theRichland farming complex in the Illinois uplands. New tree ring-based records of climate change indicate that this rapid development occurred during one of thewettest 50-year periods during the last millennium. During the next 150 years, a series of persistent droughts occurred in the Cahokian area which may be related to the eventual abandonment of theAmerican Bottom. By A.D. 1150, in the latter part of a severe 15-year drought, theRichland farming complex was mostly abandoned, eliminating an integral part ofCahokia's agricultural base. At about the same time, a 20,000-log palisade was erected around Monks Mound and theGrand Plaza, indicating increased social unrest. During this time,people began exiting Cahokia and, by the end of the Stirling phase (A.D. 1200), Cahokia's population had decreased by about 50 percent and by A.D. 1350, Cahokia and much of the central Mississippi valley had been abandoned.

Durante lafase temprana del Lohmann Mississippico (A.D. 1050-1100), el American Bottom se transformopoltiica y economi camente. Esta transformaci?n incluy? la r?pida planeaci?n de la construcci?n de Cahokia, una llegada masiva de gente al su centro, el abandono de los asentamientos de villas pre-Mississippicas, la reorganizaci?n de la agricultura en las partes bajas del no Mississippi y la fundaci?n del complejo agricola Richland en la tierras altas de Illinois. Nuevos registros de cambio climdtico, basados en anillos de drboles, indican que este rdpido desarrollo ocurrio durante uno de los mas h?medos perio dos de 50 anos del ultimo milenio. Durante los 150 anos siguientes, una serie de sequias persistentes ocurrieron en el area de Cahokia, lo que debe relacionarse con el abandono del American Bottom. Para 1150 A.D., en la ultima parte de una sever a sequia de 15 anos, el complejo agricola Richland fue casi abandonado, eliminando una parte integral de la base agricola de Cahokia. Casi al mismo tiempo, una palizada de 20,000 estacas fue levantada alrededor de Monks Mound y la Grand Plaza, indicando el incremento de conflictos sociales. Durante este tiempo, la gente empez? a salir de Cahokia y, para fines de la fase Stirling (1200 A.D.), la poblaci?n de Cahokia habia decrecido cerca del 50por ciento; para 1350 A.D., Cahokia y gran parte del volle central del Mississippi habian sido abandonados.

Recent studies have indicated a renewed and late-thirteenth-centurydecadal-scale droughts interestin thepossible influenceof climate may have impactedNative Americans acrossmuch change on precolumbian cultures (e.g., of the contiguous United States. If true, thismight Anderson et al. 2007; Benson, Petersen, and Stein not only help explain thehistorical trajectories of 2007; Benson, Berry, Jolie, Spangler, Stahle, and the ancestral Pueblo in theFour Corners area of the Hattori 2007; Cook et al. 2007; Drysdale et al. western United States, but itmight also elucidate 2006; Gill 2000; Hodell et al. 1995; Jones et al. the factors underlying the rise and fall of the 1999; Kennett and Kennett 2006; Stahle et al. Cahokians inwhat is now west-central Illinois. 2000). Benson, Berry, Jolie, Spangler, Stahle, and In a recent paper, Benson, Petersen, and Stein Hattori (2007) have suggested thatmid-twelfth (2007) compared precolumbian Southwestern

Larry V. Benson National Research Program, U. S. Geological Survey, 3215 Marine St., Boulder, CO 80303 ([email protected]) Timothy R. Pauketat Department of Anthropology, 109 Davenport Hall, University of Illinois, Urbana, IL 61801 Edward R. Cook Biology and Paleo Environment, Lamont-Doherty Earth Observatory, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964

American Antiquity 74(3), 2009, pp. 467^83 Copyright ?2009 by the Society forAmerican Archaeology

467 468AMERICAN ANTIQUITY [Vol. 74, No. 3,2009

demographic change with tree-ring-baseddrought have made use of themost recent data on building histories for theFour Corners region, reinforcing density at or aroundCahokia. On theone hand,Mil the conjectures of Judge (1989) and Douglass ner (1986, 1998) estimated theMississippian (1929) thatthe middle-twelfth- and late-thirteenth period population of a 129 square km portion of centurydroughts led to ancientmigrations. In this the ruralAmerican Bottom and also forCahokia paper, we focus on the relationship of the cultural proper. On the other hand, Pauketat and Lopinot demographic oscillation of the ancient Cahokia (1997) calculated Cahokia's pre-Mississippian and region to new tree-ring-based records of climate Mississippian-period population using structure change. In the following, we first review the cul density/phasedata fromanalyzed excavation tracts tural processes that attended the rise and fall of (15A and ICT-II). Cahokia. We then illustrateand discuss tree-ring With regard toMilner's ruralestimate, thenum based records of climate change for the region sur bers and floor areas of residential structuresat 15 roundingCahokia. Last, we compare those records sites indicate that thehighest ruralpopulation den of climate change with the cultural history of sitydated to the Stirling phase (Milner 1986). His Cahokia, demonstrating thatCahokia's "boom" later calculations of Cahokia's population were phase occurred during one of thewettest 50-year based on average structuredensities from several periods in the past millennium and that its "bust" excavated tractsat the site, some ofwhich had yet phase occurred during a series of intense and per tobe analyzed. Assuming thateach building stood sistentdroughts. only five to tenyears and was occupied by four to five people, his estimates "in the low thousands" reveal a Lohmann (Milner Cahokian Expansion and phase population peak 1998:122-124). Agricultural Reorganization By comparison, Pauketat and Lopinot's (1997) After themid-eleventh century,Cahokia was the estimate, which applies only to a 1.8 km2 high centerpiece of a large central political density occupation area ofCahokia, shows thatthe administrative complex thatalso included thepre site's population increased from 1,400-2,800 peo sent sites of St. Louis (Mound City) and East St. ple during the 50-year Edelhardt phase to Louis (Figure 1, see also Pauketat 2004). This entire 10,200-15,300 people during the subsequent 50 complex now seems theproduct of a political and year Lohmann phase. After theStirling phase, den economic expansion that occurred immediately sities declined.1 Later, Pauketat (2003:47) also prior to and in the decades following A.D. 1050 estimated thepopulation of the outlying Richland (Table 1).This development saw theabrupt planned rural farming district, covering about 300 square construction of central Cahokia centered on a sin km, based on numbers of sites and excavated build glemassive 6.5 m-high platform (thecore ofMonks ings per phase. The results of this effort indicate Mound) and theGrand Plaza (Dalan et al. 2003). isolated peak populations for that locality of At the same time, theregion saw a large-scale influx between 3,000 and 7,400 people during the late of people to "downtown Cahokia," the concomi Lohmann and early Stirlingphases. Both estimates tantabandonment of pre-Mississippian village set show that the region's peak population fallswithin tlements in theMississippi River floodplain, the theLohmann and Stirling phases. Moreover, from simultaneous dissolution of pre-Mississippian eitherpoint of view, the lowerpopulation estimates courtyards at Cahokia, and the founding of the forCahokia may be seen as "consistent with the Richland farming complex in the Illinois uplands numbers of people who could have been supported (Alt 2002; Emerson 1997a, 1997b; Lopinot et al. in this floodplain settingwith the primitive tech 1998; Pauketat 1994, 1997, 1998a, 2000, 2003; nology ofthat time" (Milner 1998:123). Cahokia, Pauketat and Lopinot 1997).We also suspect that afterall, is situated in themiddle of thewidest part an agricultural reorganization accompanied this of theAmerican Bottom's broad expanses of rela societal transformation,an argument thatrests par tivelyhigh well-drained land suitable for growing tially on regional population size and disposition. crops, a locality also rich with resources derived Although thereare several population estimates from permanent swamps and lakes (Milner and forCahokia and its immediate environs, two efforts Oliver 1999). Fish, which were readily available in Benson et al.] CAHOKIA'S BOOM AND BUST 469

kilometers

Figure 1.Major settlements in the greater Cahokia area at about A.D 1100. After Figure 6.4 in Pauketat (2007).

backwater and creek locations within theAmeri other regions of easternNorth America, therisks can Bottom, were a critical component of the ofMississippian-style agriculturalproduction were Cahokian diet (Cross 1987; L. S. Kelly 1997). The high (see Scarry 1993a, 1993b). The potential American Bottom surface-watersystems were also impacts of crop failure on local economies and home to abundant waterfowl (Kelly and Cross political organizations were great (Anderson et al. 1984). 1995; Stahle and Cleaveland 1994). Once intensi And yet, as Milner (1998) and others have fied afterA.D. 800, a whole new suite of maize argued, despite the resource richness of theAmer related agricultural and technological exigencies ican Bottom, precolumbian subsistence productiv likelymade themselves apparent (Maher 1989; itymight have been an issue of great concern to Johannessen 1984; Riley et al. 1994; Rindos and Cahokians and ruralMississippian farmers.In most Johannessen 1991). 470 AMERICANANTIQUITY [Vol. 74, No. 3, 2009

Table 1.Archaeological Chronology and Inferred Organizational Attributes for the Later Pre-Columbian American Bottom Region.

Period Phase* Years A.D. Regional-Organizational Attributes* Terminal Late Woodland Loyd 900-950 low regional population density, Cahokia is probably an ordinary village Merrell 950-1000 low regional population density, Cahokia village is first among equals, now includes uncertain number of immigrants modest regional population density, immigration continues, Cahokia assumes town-sized proportions but retains pre-Mississippian courtyard Edelhardt 1000-1050 organization

Mississippian Lohmann 1050-1100 highest regional population density, large-scale influx of immigrants, reconstruction of Cahokia according to new city plan, abandonment of pre-Mississippian settlement organization Stirling 1100-1200 sustained to slightly lower population density, homogenization of material culture, elaboration of ritual-administrative complex Moorehead 1200-1275 reduced population density, political-economic contraction or reorganization, palisade wall construction culmination of abandonment _Sand Prairie 1275-1350 low regional population density, regional emigration, following Fortier et al. 2006 **see Alt 2002; Emerson 1997a; Holley 2006; J.E. Kelly 1990; Milner 1986; Pauketat 1998a, 2003, 2004, 2005

Certainly, thehigher population estimates for the theAmerican Bottom had not transcended thecon American Bottom region, above, would have straints of isolated Mississippi River floodplain accentuated such subsistence productivity issues. localities (cf. J.E.Kelly 1990;Milner 1998; Pauke Even in recent times in theAmerican Bottom, tat 2004). Isolated single-home farmsteads were Euroamerican farmers who began their careers few and farbetween and villages or hamlets occu farming during the 1920s and 1930s, interviewed piedhigh floodplain ridges and thebluff edge of by Chmurny in 1967, were of theuniform opinion theAmerican Bottom (e.g., Emerson and Jackson that"scarcely a year passed in theBottom without 1984; J. E. Kelly 1990; Pauketat et al. 1998). some usu the occurrence of weather phenomena, Houses in these settlements, small semisubter ally unwelcome" (1973:243). Those who survived ranean pole-and-thatch huts with floors half-a over the long run were those that "divided their meter or more below ground level, were often risks," often "at the expense of increased ineffi arranged around courtyards, each centered on a ciency and lessened gains," by planting a diversity modest central marker post. The pre-Lohmann of crops inboth "well-drained and poorly-drained" phase rural settlements in theAmerican Bottom soils (Chmurny 1973:247-248). Projecting simi reveal little-to-no internal status differentiation, lar strategies and conditions into the past,Milner political specialization, communal activities, or concluded that,for ancient farmers,the "overall pic ranking (see Holley 2006; J.E. Kelly 1990; Pauke ture could have been nothing other than gloomy tat 1994). duringmuch of theMississippian period, despite It was not until the Lohmann phase that the richness of the valley. The inhabitants of the regional-organizational shiftscan be inferredfrom American Bottom were perched on the cusp settlement, monument-construction, and refuse between success and failure, and a series of bad deposition data that probably correlate with the years would have been disastrous for them" appearance of a centrally managed agricultural (1998:78). system. In the American Bottom floodplain Prior toA.D. 1050, such subsistence production proper, theMississippian transitioncorrelates with concerns might have been less problematic for abandonment of some if not many pre indigenous farmers.At that time,most analysts Mississippian villages and their replacement by would agree, thepolitical and community order in small farmsteads and large towns (Emerson Benson et al.] CAHOKIA'SBOOM AND BUST

1997a, 1997b; Mehrer 1995; Milner et al. 1984). (Alt 2001,2002; Koldehoff 1989; Pauketat 2003). All evidence todate suggests that,whether a large Richland and other upland farmersmay have town or small farmstead, the once-common pre been derivedfrom pre-Mississippian settlementson Mississippian courtyard group?single-post theMississippi floodplain or from sites in the inte houses set around an open space with a central riors of Illinois,Missouri, or Indiana (Alt 2002). marker post?disappeared ratherquickly (Pauke Radiocarbon dates togetherwith ceramic and lithic tat 1994:131-133; see also Emerson 1997a; markers indicate thatthe upland complex dates pri Mehrer 1995; Mehrer and Collins 1995; Milner marily fromA.D. 1050 to 1150 with only a few etal. 1984). buildings dating to the latteryears of theEdelhardt Emerson (1997a, 1997b) has argued that the phase (Pauketat 2003). By about A.D. 1100, each subsequent rural "dispersed communities" in the of the eight largest settlements in the Richland floodplain were articulated to larger towns and, farmingcomplex was occupied, suggesting thatthe ultimately, to Cahokia via special "nodal" farm regional population at the timewas considerably steads.At these sites, thereare new forms of ritual larger than some earlier estimates thatconsidered or special-purpose architectureand the residues of only the flood plain itself (e.g.,Milner 1998:120, various ceremonial practicesmost like thoseknown 124-125; see also Schroeder 2004). from Cahokia itself.Thus, Emerson has further Judging from archaeobotanical remains and argued, based on theirunprecedented appearance lithic-tool debris these were, first and foremost, during theLohmann phase and because of theirspe agriculturalvillages. At these sites,as at other loca cial architecture and artifacts, such nodal farm tions in the centralMississippi valley,Mill Creek steads were the "direct product of Cahokian elite cherthoe blades and thedebitage from the resharp intervention" (1997a:250). Some timenearing the ening of the same are common indicators of agri mid-eleventh century, the floodplain cultural activity (Cobb 2000; Sussenbach 1993). They are certainly prevalent at most late pre countryside about the greatmound center of Mississippian andMississippian sites in theAmer Cahokia was emptied of itsrural populations, ican Bottom region. presumably because these people moved to Mill Creek hoe blades may have been a tool largernucleated centers.With theonset of the especially necessary for the initialworking of the Lohmann phase, a new system of dispersed heavily rooted soils of theupland prairie or prairie farmsteadsand controllingnodal centerswas forest ecotone, located near most of theRichland superimposed across this depopulated land complex sites (Pauketat 2003). In fact, around scape.... I have argued, given the large Cahokia during theLohmann phase, thedensity of Lohmann and Stirling phase populations at hoe-blade resharpening flakes inexcavated domes Cahokia, that this rural organization was tic refuse indicates that the rate atwhich the tools designed to intensifythe production and mobi were used and resharpened increasedwith distance lizationof to supportthe centers [Emer staples fromCahokia at the same time thatother itemden son 1997a:250]. sities decreased. That is, the With additional surveyand excavations since the distribution of craft products and the avail 1990s, we know also know that, as part of this abilityof Cahokian subsidies or theexotic raw region-wide reorganization, an extensive upland material ... consistently falls off in the sur agricultural landscape (theRichland farmingcom roundingnorthern floodplain, decreasing fur plex) was established in the loess-covered hills east ther still as one moves out in theRichland ofCahokia proper (Alt 2001,2002; Pauketat 2003) Complex.... Conversely, the density (Figure1). [grams/cubicmeter ofdomestic refuse] ofMill Prior to theLohmann phase, therewere no set Creek cherthoe-blade fragmentsand resharp tlements of any kind across large areas of these eningflakes increases with distance from uplands. Afterward, however, a series of towns,vil Cahokia toa Richland Complex peak. In other lages, hamlets, farmsteads, and other special sites words, all other thingsbeing equal, access to appeared where none had been previously, espe Cahokian valuables and craftobjects seems to cially in theRichland and Silver Creek drainages have been inverselyrelated to the intensityof 472AMERICAN ANTIQUITY [Vol. 74, No. 3,2009

agricultural production using hoe blades theRichland farmers in such a Cahokia-centered [Pauketat 1998a:68-69, emphasis added]. process could have been to increase theproductive output of the region through regional extensifica Other upland areas in Illinois and Missouri, tion. Such extensificationmight have been neces within a 50 km radius of Cahokia, may have been sary, the productive potential of floodplain in the intensively farmed as well afterA.D. 1050; sys immediate vicinity of Cahokia possibly not being tematic archaeological surveys of this region have sufficient to provide food for a Lohmann-phase not been sufficientlyextensive to identifythe full population ofmore than "a few thousand inhabi range of Lohmann and Stirling phase sites either tants."Schroeder's (1999) revised productivityesti along theKaskaskia River to the east or the lower mates for precolumbian North American maize Missouri River to thewest (Blake 1955; Hargrave production also supports an argument, still tenta et al. 1983; Harl 1991; Hunt 1974; Kuttruff 1972). tive, thata Cahokian population of 10,000 ormore In any event, theunprecedented Richland complex people during theLohmann phase might have had occupation of theuplands?thousands of possible difficulty feeding themselves, particularly in bad immigrants and other resettled farmers?may be years, without the Richland complex producers related to their proximity to highly productive (see also Pauketat 2003:48). prairie and prairie-edge soils at theheadwaters and interfluvesof Richland and Silver Creeks. The tall of theAmerican Bottom and grass prairie was so nitrogen rich that fertilizers Depopulation were not needed to sustain a luxuriant crop for Abandonment of the Richland Farming some time; sometime prior toA.D. 1860, a traveler Complex noted that "I saw fields of maize thathave been After the Lohmann phase, the population of the grown for 30 years and that, too,without any fer Cahokia region decreased as some residents appear tilizer" (Sutton 1976: 202). These soils contained to have relocated elsewhere. Between 5,200 and somuch nitrogen thatearly Illinois farmers found 7,200 people are estimated to have occupied prairie soil toorich togrow wheat (Welch 1979:10), Cahokia during the twelfth-centuryStirling phase, and in latertimes, nitrate poisoning of livestockwas dropping to between 3,000 and 4,500 people dur a widespread problem (Davidson et al. 1941). Dur ing the thirteenth-century Moorehead phase ing the 1800s a drought thatkilled corn inmid (Pauketat and Lopinot 1997). Settlement patterns season allowed the plant stalks to accumulate also changed, with late Stirling- andMoorehead nitrogen to thepoint that they"burned rapidly like phase houses becoming increasingly clustered into the fuse of a fire cracker" (Mayo 1895:5). Indeed, house lots (Collins 1997;Mehrerand Collins 1995; such soils are closer here toCahokia thananywhere Pauketat 1998b). else in the region. Such proximitymay not have Whereas the rural population on theMissis been lost on Cahokians, as Alt (2006) has recently sippi River floodplain proper appears to have noted by her identificationof a "nodal village" or increased slightlyduring the twelfthcentury (Mil "ritual outpost" ofCahokia in the formof theoddly ner 1998), by the late Stirling phase (A.D. compact multi-building hilltop site ofGrossmann. 1150-1200) almost all Mississippian sites in the When excavated in its this site, located 16 entirety, Richland were abandoned km east of Cahokia in themidst of theRichland farming complex (Pauketat 2003). In addition, sites evidence of ritual Stirling-phase complex, produced atypical in theAmerican Bottom were located at lower ele events and architectureand artifacts Cahokia-style vations thanearlier Lohmann and laterMoorehead (Alt 2002, 2006). Based on such findings, along sites (Emerson 1992). This might suggest that a with the artifact-densitypatterns noted above, it relative drop in thewater table accompanied the seems plausible that the Richland complex may cultural changes of the times, a concept supported well have been an organized farmingdistrict, min by zooarchaeological data that indicate fewer fish istered to or administered by Cahokians. remains associated with Stirling-phase sites (L.S. One might even suspect that a populous Rich Kelly 1997). Possibly some ormost of theAmer land farming complex may have been an integral ican Bottom's lakes and sloughs desiccated dur aspect of the earlyMississippian reorganization of ing the twelfthcentury. By theMoorehead phase, agricultural production in the region. The role of Benson et al.] CAHOKIA'S BOOM AND BUST 473

the rural population density, in the flood plain, work used for PDSI reconstruction over North dropped sharply (Milner 1986). America originally was initially composed of 835 It seems that, during the twelfth century, the annually resolved records.A revised network con "increasingly top-heavy economy and the taining 1,825 records is now available and has been monument-laden spaces of Cahokia grew out of used in thispaper. The PDSI data used in thispaper proportion to the shrinking farming population" were calibrated using a point-by-point regression (Pauketat 2004:152). There are signs of alterations of prewhitened instrumentalPDSI data against tree in thecentral appropriation of labor, ifnot also indi ring records for theperiod 1928-1978 (Cook et al. cations of social unrest. For instance, the con 2007). structionof Monks Mound seems to have ceased In a nested reconstructionprocedure, thePDSI by the end of the early Stirling phase (aboutA.D. reconstructions are progressively pushed back in 1150) and most other mounds with known con time by dropping out shorter series. This method struction dates have only Stirling- and not of extending the reconstructionsback in timewas Moorehead-phase construction fills (Pauketat evaluated in detail by Cook et al. (2004) in their 2004; Reed et al. 1968). Even more important,at "Supporting Online Material" and was found to about A.D. 1150, thefirst of several 3-km-long 20 work well over the full 286-point grid. The Table ft-high palisade walls was constructed around 2 results for grid points 209 and, especially, 210 Monks Mound and theGrand Plaza (Iseminger et strongly support the validity of the nested recon al. 1990). Similar walls probably went up else structionprocedure described in theSOM. InTable where in the region, with theEast St. Louis site 2, the relevantmodels for grid points 209 and 210 being the best documented instance (Pauketat for the Lohmann and Stirling phases (A.D. 2005). 1050-1200) are indicated. IFYR is thefirst year of In any event, the remaining Mississippians each model and NTR is the number of tree-ring appear tohave leftthe Cahokia area during the late chronologies available for thatmodel. RSQ is the thirteenthand early- fourteenthcenturies; noMis calibrated variance (1928-1978 calibration period) sissippian cemetery is known to post-date A.D. and CVRE is the cross-validation (leave one out) 1300 (Emerson andHargrave 2000; Emerson et al. reduction of error in the calibration period. Both 2003). Moreover, theabandonment ofCahokia was parameters decrease as thenumber of chronologies not an isolated phenomena. By theend of the four decrease; however, all are significantat/? < .01.For teenthcentury, the fortifiedcenters and their sup the 1900-1927 verification testperiod (data with port settlementsaround theOhio-Mississippi river held from the calibration exercise), R2 (square of confluence were also abandoned (Cobb and But thePearson correlation), RE (reduction of error), ler 2002; Williams 1990). and CE (coefficientof efficiency) are used. All are positive and highly significant (p < .01).These ver ification tests are measures of Climate Change and its Impact on the highly rigorous reconstruction and as such indicate that Cahokian Polity "accuracy" the reconstructions are likely to have significant Recently, Cook et al. (2004) published summer "hindcast" skill in thepre-1200 period. The strength (June-July-August)Palmer Drought Severity Index of this skill is of course not perfect,but its level is (PDSI) values for gridded locations across much comparable to other high-quality drought and ofNorth America. The PDSI value is ameasure of streamflow reconstructions developed from tree available soil moisture calculated frommonthly rings for thisregion (e.g.,Biasing andDuvick 1984; temperatureand precipitation.Negative PDSI val Stahle and Cleaveland 1992,1994). The compara ues indicate dry conditions, whereas positive val bilityof RE andCE withRSQ forgrid point 210 ues indicate wet conditions. This index was also indicates thatthe reconstruction is excellent in specifically designed to evaluate drought impacts quality and nearly equal in strengthover time.Grid on agriculture (Palmer 1965); PDSI values range point 209 is less so, but still significant (see Cook from -6 (extreme drought) to +6 (extremewet). et al. [2004, 2007] for details of thePDSI recon The drought reconstructionsare based on a 286 structions). point grid of instrumentalPDSIs. The tree-ringnet Two PDSI records, one at 90?W 37.5?N (site 474 AMERICAN ANTIQUITY [Vol. 74, No. 3, 2009

Table 2. Calibration and Verification Statistics for Tree-Ring-Based PDSI Reconstructions

_FYR_NTR_RSQ CVRE_#_RE_CE GRID POINT 209 1190 3 .450 .415 .406 .199 .198 1140 2 .342 .280 .333 .164 .164 920 1 .299 .237 .275 .174 .173 GRID POINT 210 1190 7 .524 .485 .614 .542 .542 1150 6 .492 .450 .637 .593 .593 1140 6 .492 .450 .637 .593 .593 1020 3 .385 .340 .669 .637 .637 1000 2 .297 .246 .524 .441 .441 _920_1_,240_ATS_.264 .170 .170 = = = FYR first year of each model; NTR number of tree-ring chronologies; RSQ calibrated variance of the 1928-1978 cal = = = ibration period; CVRE cross-validation reduction of error; R2 square of the Pearson correlation; RE reduction error; = CE coefficient of efficiency.

209) and the other at 90?W 40.0?N (site 210) (Figures 3 and 4). (NOAA 2007) bracket Cahokia (Figure 2). The InFigure 5, thePDSI record forwest-central Illi mean PDSI data for the two grid points are shown nois (Figure 5a) is compared with estimates of pop inFigure 3. Trees used in thePDSI constructions ulation change for downtown Cahokia and the for sites 209 and 210 include Bald cypress (Tax Richland farmingcomplex (Figure 5c). Also shown odium distichum) and red cedar (Juniperus vir are approximate timesof the initialconstruction and giniana), whose growth are highly correlatedwith reconstructionof thepalisade wall thatsurrounded spring rainfall (see, e.g., Stahle and Cleaveland Monks Mound and theGrand Plaza (Figure 5b). 1992). According to current archaeological recon The stacked (mean of the two data series) PDSI structions,a major political upheaval occurred at record indicates that theLohmann phase was rela Cahokia at the beginning of theLohmann phase, tivelywet, comparable to thewettest periods occur involving a rapid ascendancy of a new formof cen ring during the past 1,000 years (Figure 3a).2 On tralized authorityand political community (Pauke the other hand, the Stirling phase and thefirst half tat 1997, 2004). During this "boom" period, of theMoorehead phase were characterized by a political consolidation was accompanied by the series of droughts thatincreased in intensityand per immigration of several thousand individuals to sistency during the subsequent 145 years (Figure downtown Cahokia and the surrounding country 3b). Annual PDSI data indicate positive (wet) val side. ues for 28 out of 50 years during the Lohmann However, for Cahokia to grow, agricultural phase, 18 out of 50 years for theearly Stirlingphase, intensification or extensification must have and only 11 out of 50 years for the late Stirling occurred and, clearly, the environment, including phase (Figure 3c). These data indicate thatdrought climate, established the limits of intensification. occurred frequentlyduring the early Stirling phase Cahokians, or various affiliated farming popula and that itdominated the late Stirling phase. tions,might have provided the impetus. In either The areal extent of the two multi-decadal case, one could argue that,to a firstapproximation, droughts that followed theA.D. 1050-1100 wet therewere limits to theproductive potential of the period are shown in Figure 4. The average PDSI floodplain aroundCahokia, particularlywith regard value for thepast thousand years at grid points 209 tomaize (seeMilner 1998; Schroeder 1999). Given = and 210 (PDSI -0.4) is used to form thebound Pauketat and Lopinot's (1997) estimate that ary between wet and dry regions. The droughts Cahokia contained between 10,200 and 15,300 recorded at grid points 209 and 210 were regional people during theLohmann phase, some and prob in extent and impacted much of theMidwest as ablymuch ofCahokia's agricultural supportmight well as major surface-water drainages that flow have come from outside theAmerican Bottom into and along theAmerican Bottom. It is evident (Pauketat 2004:106). that west-central Illinois experienced severe The Richland farming complex represents the drought 140 out of 145 years followingA.D. 1100 most likelyengine thatdrove Cahokia's agricultural CAHOKIA'SBOOM AND BUST

-91.5 -91 -90.5 -90 -89.5 -89 -88.5 -88 -87.5

Longitude

Figure 2. Locations of PDSI sites 209 and 210 relative to Cahokia. reorganization of its countryside, doubtless sup might have been unable to absorb its Lohmann plemented by theproduce of others districtswithin phase immigrantpopulations. Given above normal theAmerican Bottom and beyond. Perhaps thereor rainfall, the upland farming sites were probably ganization of theAmerican Bottom may also rep able to provision Cahokia with a surplus ofmaize resent a response towetter conditions thatbegan and other crops to use for ceremonial feasts or to before theLohmann phase; i.e., increased precip fill central storehouses for lateruse (e.g., Pauketat itation and flooding of theMississippi River and et al. 2002). its side tributariesmay have entailed a rise in the The freeze-freegrowing season is typically long elevation of the local groundwater table. Shallow inwhat is now west-central Illinois (Joos 1960); groundwater in theAmerican Bottom will tend to therefore,maize production ismostly dependent on be anoxic, given thehigh concentration of organic soilmoisture during thegrowing season. In the Illi carbon inbottom sediments.Maize cannot survive nois highlands, soilmoisture represents a balance in such an environmentbecause itsroots need oxy between winter precipitation thatencourages ger gen. Thus, farmersmay have moved to higher mination, summerprecipitation thatpromotes plant ground in response to the loss ofwell-drained agri growth, and evaporation / evapotranspiration that cultural bottomland. removes soilmoisture during the growing season. Without theRichland complex, greaterCahokia The American Bottom may flood in the spring or 476AMERICAN ANTIQUITY [Vol. 74, No. 3,2009

10501100 11501200 CALENDAR A.D.

Figure 3. Tree-ring-based reconstructions of the Palmer Drought Severity Index (PDSI) for sites 209 and 210 (PDSI Site Map) bracketing the Cahokia area for the period (a) A.D. 1000 -2000, (b) A.D. 1000-1275, and (c) A.D. 1050-1200). A PDSI value of -.4 (mean value for last 1,000 years) was used to separate wet from dry periods. Positive values indicate = wet conditions; negative values indicate drought conditions. E pre-Mississippian Edelhardt phase, and L, S, M, and = SP Mississippian Lohmann, Stirling, Moorehead, and Sand Prairie phases.

early summer, increasing soil moisture; however, occurred that became more persistent with time additional summer rains are stillneeded formaize (Figure 3a, b).We believe thatabandonment of the to reachmaturity. Richland farming complex by about A.D. 1150 Beginning in the Stirling phase and continuing (Figure 5b) may be related to these droughts, and until about A.D. 1250, a series of intensedroughts thus that the agricultural "glue" that held the Benson et al.] CAHOKIA'S BOOM AND BUST 477

20

-130 -120 -110 -100 -90 -80 -60 -70

20

-130 -120 -110 -100 -90 -80 -60 -70 LONGITUDE

Figure 4. PDSI contour maps for two 70-year periods following A.D. 1100. A PDSI value of -0.4 (mean value for the period A.D. 1000 to 2000) is used to divide drought areas (medium gray to black) from relatively wet areas (light gray).

Cahokian polity togethermight have dissolved. and local laborers had to eat. In fact, a question Cahokia's establishment and rapid expansion at remains as to whether the Cahokian leadership the beginning of theLohmann phase (Figure 5b) established a program of agricultural intensifica certainly evidences great organizational change tion or whether agricultural intensificationwas a and, perhaps, political will on the part of its consequence of the in-migration of people with founders.But whatever the factors thatdrew thou previous agricultural experience. sands of people toCahokia (e.g., charismatic lead The latterpart of the Edelhardt phase (A.D. ership,a religious cult,a more comfortable lifestyle, 1030-1040) was extremelywet (Figure 5a), and it a Pax Cahokiana a 'la Pauketat [2007]), immigrants may be thatpre-Mississippian farmers in theUli 478 AMERICANANTIQUITY [Vol. 74, No. 3, 2009

1000 1050 1100 1150 1200 1250 Calendar A.D.

Figure 5. (a) A 7-year running average of the tree-ring-based PDSI record for sites 209 and 210 for the period A.D. 1000-1275 compared with (b) population estimates for Cahokia (Pauketat and Lopinot 1997), and population estimates for the Richland farming complex (Pauketat 2003). The vertical dotted lines indicate phase and intra-phase boundaries. The filled circles represent maximum population estimates and the open circles indicate minimum population estimates. Solid lines connect the population estimates for Downtown Cahokia (DC) and the dashed lines connect the population estimates for the Richland Complex (RC). The dark solid vertical rectangle indicates the initial construction of the Cahokian palisade and the lighter rectangles indicate schematically its subsequent reconstructions.

nois uplands were able to produce a series of good ure 3c), leaving Cahokia without an integralpart crops during thatperiod, suggesting a location and of its subsistence base. Even droughts thatoccur procedure for future agricultural intensification. today decrease maize yields; the 1950s drought However, itwas impossible for theCahokian lead thataffected much of thecontiguous United States, ership to forecast the intensityand duration of the including west-central Illinois, greatly reduced anomalously wet period thatfollowed A.D. 1050. maize yields in both counties (Madison and St. We, therefore,suggest thatcontinued immigration Clair) inwhich theCahokians and Richland farm or localized population growthduring theLohmann ers once resided (Figure 6). Obviously modern phase might have been an iterative response to day maize production differs radically from nearly continuous agricultural success experienced Mississippian production; however, reduction in by local farmers,especially thosemoving into the maize yields duringdrought inmodern times serves Richland farmingcomplex. That is, the expansion to demonstrate the fragilityof maize production of productive upland farmingwas able tokeep pace even in thepresence of improved technologies. with immigration,encouraging thecontinued influx The loss of an integralpart of itssubsistence base of individuals to greaterCahokia. need not have entailed a loss of control on thepart Between A.D. 1100 and 1245, when drought of Cahokians during the relatively dry and agri became the rule and not the exception, a popula culturallymarginal early Stirling phase; but even tion thathad been farmingupland tallgrass prairie ifCahokians attempted tomaintain the social and met climate realityhead on; theRichland farming political statusquo, theywould have done so in the complex was abandoned by A.D. 1150 (perhaps face of people leaving Cahokia and, quite likely, spurredby a severe 15-year-longdrought, see Fig the region surroundingCahokia. Benson et al.] CAHOKIA'S BOOM AND BUST 479

1945 1950 1955 1960 1965 1970 1975 1980 1985 Calendar AD

Figure 6. (a) Detrended maize yields for St. Clair county, Illinois; (b) annual precipitation received at Belleville, Illinois; and (c) detrended maize yields forMadison county, Illinois. Detrending was accomplished by fitting a second-degree polynomial to maize yields for the period 1918-2006. The detrending was done in order to remove the continuously increasing trend inmaize productivity due to fertilization and the introduction of new maize hybrids. The time period displayed (1948-1982) was chosen to highlight the 1950s drought as well as less-intense droughts that occurred within the subsequent three decades. Gray-toned areas indicate times of drought and reduced maize yields.

Conclusions higher ground inan effortto avoid a rising ground water table thatwas responding to anomalously Cahokia's cultural and demographic development high precipitation as well as to frequent flooding between A.D. 1000 and 1300 was perhaps condi of theMississippi River and its side tributaries. tioned by a volatile climate. The latterpart of the Midway through the century,a regional reorgani pre-Mississippian Edelhardt phase and the entire zation centered on Cahokia sealed thedeal. While Lohmann phase were characterized by extremely abundant precipitation may have diminished the wet conditions (Figure 5a). The population of agricultural potential of theAmerican Bottom, it downtown Cahokia increased from 1,400-2,800 allowed the agricultural intensificationof the Illi people during the pre-Mississippian Edelhardt nois uplands and thefounding of theRichland farm phase (A.D. 1000-1050) to 10,200-15,300 people ing complex (Figure 5b). In addition, the wet during the early Mississippian Lohmann phase climate fed themarshlands, streams, sloughs, and (A.D. 1050-1100 (Figure 5b). Between A.D. 1000 lakes in theAmerican Bottom, thereby increasing and 1100, American Bottom farmersmoved to its fishery and furtherproviding an environment 480AMERICAN ANTIQUITY [Vol. 74, No. 3,2009

and Paul A. conducive to theexploitation ofwildfowl and small Mayewski 2007 Climate and Cultural Change: Exploring Holocene mammals. Thus, an overall increase in agricultural Transitions. In Climate Change and Cultural Dynamics: productivityand thefecundity of theAmerican Bot A Global Perspective onMid-Holocene Transitions, edited David G. Kirk A. and Daniel H. tom allowed for the rapid expansion of population by Anderson, Maasch, 1-23. Academic London. in downtown Cahokia and in administra Sandweiss, pp. Press, adjacent Anderson, David G, and David W. Stahle, and Malcolm M tive centers. Cleaveland 1995 Paleoclimate and thePotential Food Reserves ofMis A series of severe and persistent regional-scale Societies: A Case from the Savannah occurred between A.D. 1100 and 1245 sissippian Study droughts River Valley. American Antiquity 60:258-286. were (Figure 5a). Stirling-phase farms established Benson, Larry, and Kenneth Petersen, and John Stein at low elevations in theAmerican Bottom, sug 2007 Anasazi (Pre-Columbian Native-American) Migra tions during theMiddle-12th and Late-13th Centuries: Were thatdrought had caused thewater table to gesting They Drought Induced? Climatic Change 83:187-213. on fall.The ruralpopulation theMississippi River Benson, Larry V., and Michael S. Berry, Edward A. Jolie, Jerry D. David W. and M. Hattori floodplain increased slightly during the Stirling Spangler, Stahle, Eugene 2007 Possible Impacts of Early-ll01-, Middle-1201-, and indicating thatagricultural intensi phase, perhaps Late-13th-Century Droughts onWestern Native Americans was fication of theAmerican Bottom occurring in and theMississippian Cahokians. Quaternary Science response to drier conditions or because of the relo Reviews 26:336-350. Leonard W. cation of farmersdown onto the Blake, previously upland 1955 The Lambert-St. Louis Airport site. The Missouri are floodplain. Fewer fish remains associated with Archaeologist 17:27-39. T. and D. N. Duvick Stirling-phase sites, also suggesting thatdrought Biasing, J., 1984 Reconstruction of in theNorth also led to the seasonal desiccation of and Precipitation History sloughs American Corn Belt Using Tree Rings. Nature lakes in theAmerican Bottom. But perhaps the 307:143-145. William W. greatest impact of droughtwas thenearly complete Churmny, 1973 The Ecology of theMiddle Mississippian Occupation abandonment of theRichland farmingcomplex by of theAmerican Bottom. Unpublished Ph.D. dissertation, about A.D. 1150 (Figure 5b). Persistent drought Department of Anthropology, University of Illinois, Urbana. appears tohave led to thedownfall of upland farm Charles R. ifnot also to the demise of Cahokia. Cobb, ing 2000 From Quarry toCornfield: The Political economy of Mississippian Hoe Production. University of Alabama The authors wish to thank Charles Acknowledgments. Cobb, Press, Tuscaloosa. Gregory Waselkov, Eugene Hattori, Ken Petersen, Howard Cobb, Charles R., and Brian M. Butler Taylor, Gary Landis, and four anonymous reviewers for their 2002 The Vacant Quarter Revisited: Late Mississippian Lower Ohio American comments and suggestions on earlier versions of thismanu Abandonment of the Valley. Antiq 67:625-641. script. Parts of the archaeological research used in this paper uity Collins, JamesM. were funded by the National Science Foundation (BNS 1997 Cahokia Settlement and Social Structures as Viewed 9305404, SBR 99-96169), National Geographic Society, from the ICT-II. In Cahokia: Domination and Ideology in Wenner-Gren Foundation forAnthropological Research, the theMississippian World, edited by Timothy R. Pauketat Illinois Archaeological Research Program at Transportation and Thomas E. Emerson, pp. 124-140. University of of and the Illinois of the University Illinois, Department Nebraska Press, Lincoln. errors and omissions of fact are the sole Transportation. All Cook, Edward R., and Richard Seager, Mark A. Cane, and responsibility of the authors. David W. Stahle 2007 North American Drought: Reconstructions, Causes, and Consequences. Earth-Science Reviews 81:93-134. References Cited Cook, Edward R., and Connie A. Woodhouse, C. Mark Eakin, David M. Meko, and David W. Stahle Alt, Susan M. 2004 Long-Term Aridity Changes in theWestern United 2000 Cahokian Change and theAuthority of Tradition. In States. Science 306:1015-1018. Supporting Online Mate The Archaeology ofTraditions: Agency and History before rial accessed online at http://www.sciencemag.org/cgi/con and after Columbus, edited by Timothy R. Pauketat, pp. tent/full/1102586/DCl). 141-156. University Press of Florida, Gainesville. 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southeastern U.S. for the past 1000 years. Bulletin of the Notes American Meteorology Society 73:1947-1961. David and Malcolm K. Cleaveland Stahle, W., 1. There are differences in the population estimation pro 1994 Reconstructed Rainfall Over the South Tree-ring cedures, but it is possible to reconcile the two estimates cited eastern U.S.A. during theMedieval Warm Period and Lit here. Pauketat and Lopinot (1997) use the commonly tle Ice Age. Climatic Change 26: 199-212. accepted duration of 50 years for the Lohmann phase and Stahle, David W, and Edward R. Cook, Malcolm K. Cleave believe 10 years to have been a minimum uselife for land, Matthew Therrell, David Meko, Henri Grissino average houses at Cahokia; Milner assumes a Mayer, Emma Watson, and Brian Luckman (1998:123) century duration for the Lohmann and considers 10 2000 Tree-ring Data Document 16thCentury Megadrought long phase years over North America. Eos: Transactions of theAmerican a maximum house-duration estimate. More recently, Pauketat Geophysical Union 81(21):121-125. (2003) argued that both the 50-year phase length and the 10 Tom Sussenbach, year building-duration estimates best fit the archaeological 1993 Agricultural Intensification andMississippian Devel evidence of numbers of domiciles and their rates of recon opment in theConfluence Region of theMississippi River struction at Cahokia and other excavated sites in the region. Valley. Unpublished Ph.D. dissertation, Department of In the end, ifwe alterMilner's Lohmann phase duration to 50 Anthropology, University of Illinois, Urbana. there is "substantial between these two Sutton, Robert P. years, agreement" estimates 1860 The Prairie State. A Documentary History of Illinois: (Pauketat 2003:47). 2. The boundaries between are not Colonial Years to I860. William B. Eerdmans Publishing archaeological phases Company, Grand Rapids, Michigan. nearly as precise as the tree-ring-based reconstructions of the L. Fred Welch, PDSI which are accurate to one year. However, the timing of 1979 Use and Behavior in Production. Illi Nitrogen Crop the phase boundaries and the durations of the phases are gen nois Agricultural Experiment Station Bulletin 761, Urbana. erally accepted by Mississippian scholars and may be accu Williams, Stephen rate to ?25 years. 1990 The Vacant Quarter and Other Late Events in the Lower Valley. In Towns and Temples Along theMississippi, Submitted September 3, 2007; Revised August 1, 2008; edited by David H. Dye and Cheryl Cox, pp. 170-180. Uni 18, 2009. versity ofAlabama Press, Tuscaloosa. Accepted May