FORESTS, FIELDS, AND FLOODS: ENVIRONMENTAL CHANGE AND THE RISE AND FALL OF CAHOKIA (Invited Presentation)

In the 11^th century, Cahokia emerged as a major agricultural and political center in the floodplain of the central Mississippi River. By the mid-14^thcentury, however, Cahokia and the surrounding area were almost completely abandoned. The relative importance of sociopolitical and environmental factors in shaping the history of Cahokia is debated by archaeologists and geoscientists. Here, I present a series of paleoecological and paleohydrological records from floodplain lakes in the Cahokia region to examine the degree to which Cahokians modified their environment, and the effects of environmental changes on regional population dynamics.

Based on fossil pollen assemblages, macroscopic charcoal, and stable carbon isotopes (δ¹³C_org), 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 at A.D. 900, and well before the emergence of Cahokia as a regional center at A.D. 1050. Prehistoric human impacts to the environment may have 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 environmental degradation motivated individuals to abandon Cahokia.

Synchronous shifts in sediment composition and particle-size observed in cores from two oxbow lakes are consistent with deposition of fluvial sediments following inundation of the floodplain by the Mississippi River. At least eight high-magnitude floods are identified over the last 1,800 years. No large floods occurred from A.D. 600–1200, 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 at 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 were, in part, societal responses to shifts in hydrological conditions caused by late Holocene climate variability.