Precipitation reconstructions based on bald cypress (Taxodium distichum L. Rich) annual growth ring data collected from locations near the Savannah River valley, coupled with a series of simple models of storage capability, are used to calculate the agricultural food reserves potentially available each year from A.D. 1005 to 1600 to local prehistoric Mississippian populations. The resulting food reserve estimates suggest that interannual variation in rainfall during the growing season may have resulted in both extended periods of food surplus and food shortfall. We hypothesize that prolonged episodes of agricultural food surplus and shortfall had a pronounced impact on the historical trajectories of these chiefdom societies. This argument is supported by historical accounts describing the impact of drought during the period of Spanish settlement at Santa Elena (A.D. 1565-1587), and offers a possible explanation for some of the major changes observed in the late prehistoric archaeological record in the Savannah River valley, including the emergence, expansión, and decline of several mound centers and the eventual abandonment of a large portion of the basin. The study indicates the value, and potential, of analyses linking archaeological, historical, and dendrochronological data in the southeastern United States.