Sequential studies of benthonic foraminifera in the Miramichi estuary, New Brunswick over a 12 year period indicate that a "transitional" foraminiferal assemblage, which defines a zone of interaction of river and open bay environmental factors, has developed in Miramichi Inner Bay since 1964. Abundance variations of the transitional foraminiferal indicator species Ammotium cassis relative to Miliammina fusca (upper estuarine indicator) and Elphidium excavatum forma clavata (marginal marine indicator) in short cores suggest that the regional extent of this transitional thanatotope in the western part of Miramichi Inner Bay has varied considerably during the past 80–100 years. These changes appear to be related to total annual river discharge variations, competence of the Miramichi River system, and. possibly, changes in the tidal circulation pattern of bay water. Tidal circulation may have been altered by the location and cross-sectional area of several channels that occur between the barrier islands on the seaward side of the bay.