ABSTRACT:The Susquehanna River Basin (SRB) is an extensive river drainage network that covers a total of 71 228 km 2 in portions of New York, Pennsylvania, and Maryland, with more than 4 million inhabitants. It is the major source of fresh water to the USA's largest estuary, the Chesapeake Bay. Therefore, the hydroclimatic conditions within the SRB affect a large human population in addition to the ecological health of one of the world's most complicated and important estuarine environments.This research project seeks to document the hydroclimatic history of the SRB and to understand the relationships between global-scale forcing mechanisms and the climatic variability of the Basin. Hydroclimatic variability since 1680 is documented through the reconstruction of temperature, precipitation, streamflow and a drought index using proxy and observational data. Results indicate that the hydroclimatic history of the SRB is very complex. Prior to the advent of European settlement, the SRB seems to have responded in concert with other landmasses within the North Atlantic Sector evidencing a regional manifestation of both the Medieval Warm Period and the Little Ice Age. Since the beginning of European settlement, the hydroclimate of the SRB has varied greatly on both inter-annual and decadal time scales. The climatic variability of the SRB is found to be only weakly associated with large-scale forcing mechanisms that are commonly assumed to influence the eastern USA. A notable exception is a strong temperature response associated with volcanic activity. Thus, regional forcings and land cover variations may play an important role in the hydroclimatic variability of this important river system.