Results of a detailed oceanographic survey in the Saint John River Estuary, New Brunswick, Canada are presented. It is shown that interfacial mixing occurs in this highly stratified basin at discrete locations at a particular phase of the tide leading to a plunging pycnocline at most of these locations. This process is possibly initiated by different kind of internal waves or/and the changing velocity direction on interaction with the irregular bathymetry. Analysis of the flow structure in

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... of the critical Richardson number, based on previous studies in the Baltic Sea, shows that these mechanisms and similar phenomena evidently occur at strong density interfaces in the Baltic Sea area where internal waves are less regular but still essentially contribute to interfacial mixing.