Regional Migration and Cahokian Population Change in the Context of Climate Change and Hydrological Events

We explore the population history of Cahokia relative to local and regional climatological conditions to better understand the factors that may have contributed to the migration of people into and out of the American Bottom. Paleoenvironmental proxies from two sediment cores from Horseshoe Lake, Illinois, are integrated with fecal stanol data from the same cores to investigate the relationships among environmental events, climate change, and population size for Cahokia and the Horseshoe Lake watershed between ca. A.D. 975 and 1350. There is a significant decrease in population within the Horseshoe Lake watershed after a major flood event and the onset of decreased summer rainfall ca. A.D. 1160. Based on the timing of the appearance of Cahokian ceramics at sites across the Midwest and distinctive non-Cahokian pottery at sites across the American Bottom, migration into and out of Cahokia predates these climatic changes, which indicates that political, economic, social, genealogical, religious, and historically contingent concerns played greater roles in the rise of Cahokia and in the development of external relationships. Population decline accelerates rapidly after the flood and onset of decreased summer rainfall, and we suggest this decline and associated sociopolitical changes at Cahokia were induced by a complex interplay of these environmental events and regional climate change with changing genealogical, social, political, religious, economic, ecological, and historically contingent considerations at Cahokia. As population declined at Cahokia, the distribution of distinctive material culture indicates that external relationships to the north and west shrank and pivoted to the trans-Mississippi South and Southeast.