Tidal models for the Bay of Fundy, Canada — site of the highest recorded modern tide — show that tidal amplification began in the early Holocene and by ca. 5000 BP the range was almost 80% of the present range. Empirical data consisting of 146 sea-level index points and other observations appear to contradict model results. Aggregated relative sea-level data for Chignecto Bay and Minas Basin show that rapid tidal expansion began ca. 3400 BP. However, if we separate these two geographically separate data sets, evidence for this rapid late-Holocene tidal expansion is confined to Minas Basin. We explain this singularity by positing a barrier at the mouth of Minas Basin, at the Minas Passage, that delayed tidal expansion. With the rapid breakdown of this barrier and near-instantaneous tidal expansion, water temperature dropped, tidal currents and turbidity increased, and the form of the inner estuary was changed from lagoonal–mesotidal to macrotidal. We argue that the catastrophic breakdown of the barrier is related in the aboriginal legend of Glooscap, showing that aboriginal peoples observed the rapid environmental changes and preserved an oral record for 3400 years.