Forests, Fields, and Floods: A Historical Ecology of the Cahokia Region, Illinois, USA

The decline and abandonment of Cahokia (A.D. 1050--A.D. 1350), a major Late Prehistoric population center in the central Mississippi River valley, has previously been attributed to climatic variability, resource overexploitation, warfare, sociopolitical factionalism, and other factors. Lacustrine sedimentary records offer a means to reconstruct past environmental changes at local- to regional-scales, but few such paleoenvironmental records have been available to understand the role of environmental change in Cahokia's emergence and decline. This dissertation presents multi-proxy sedimentary records from three oxbow lakes in the central Mississippi River valley to establish an ecological and hydro-climatic history of the Cahokia region.;Based on fossil pollen assemblages, charcoal, and stable carbon isotopes, the widespread removal of trees and the expansion of croplands began at the onset of the Late Woodland period (ca. A.D. 400), centuries before the widespread use of maize (Zea mays subsp. mays) in this region around A.D. 900, and well before the emergence of Cahokia as a regional center at A.D. 1050. The overexploitation of woodland resources near Cahokia and neighboring population centers in the likely played a role in the agricultural intensification and regional trade patterns that characterize the emergence of Cahokia, but the paleoecological data do not support the hypothesis that severe deforestation alone motivated individuals to abandon Cahokia.;Shifts in sediment composition and particle-size observed in cores from two oxbow lakes are consistent with deposition of floodwater sediments following inundation of the floodplain by the Mississippi River. At least eight high-magnitude floods are identified over the last 1,800 years, with large floods occurring about once a century between A.D. 300--A.D. 600 and A.D. 1200--A.D. 1850. No large floods occurred from A.D. 600--A.D. 1200, coinciding with a period of midcontinental aridity, together with population growth and agricultural intensification in the floodplain of the central Mississippi River valley. The onset of Cahokia's depopulation and sociopolitical fragmentation around A.D. 1200 coincides with the return of large floods as midcontinental aridity waned. These findings imply that the emergence, decline, and abandonment of Cahokia may be, in part, societal responses to shifts in hydrological conditions caused by climatic variability.