The dispersal of Iroquoian groups from St. Lawrence River valley during the 15th and 16th centuries A.D. has been a source of archaeological inquiry for decades. Social network analysis presented here indicates that sites from Jefferson County, New York at the head of the St. Lawrence River controlled interactions within regional social signaling networks during the 15th century A.D. Measures indicate that Jefferson County sites were in brokerage liaison positions between sites in New York and Ontario. In the network for the subsequent century, to which no Jefferson County sites are assigned, no single group took the place of Jefferson County in controlling network flow. The dispersal of Jefferson County populations effectively ended this brokerage function concomitant with the emergence of the nascent Huron-Wendat and Iroquois confederacies and may have contributed to the escalation of conflict between these entities. These results add to a growing literature on the use of network analyses with archaeological data and contribute new insights into processes of population relocation and geopolitical realignment, as well as the role of borderlands and frontiers in nonstate societies.