Properties of a topographic wave are calculated for an idealized shore zone model simulating the north shore of Lake Ontario. Observations at the Oshawa “coastal chain” taken during the International Field Year on the Great Lakes are then scrutinized, concentrating on three flow-reversal episodes involving deep, barotropic currents. It is shown that the observed phenomena have characteristics very similar to those of the idealized topographic wave. A legitimate interpretation of the observed 12–16 day periodicities in longshore flow within the coastal zone of Lake Ontario is that they are caused by a combination of internal Kelvin waves and topographic waves. Although the speeds of these two kinds of vorticity waves are very similar in Lake Ontario, a careful consideration of the evidence clearly shows phase separation, particularly so late in the season when reduced density differences lead to a slowing down of internal Kelvin waves. Abstract Properties of a topographic wave are calculated for an idealized shore zone model simulating the north shore of Lake Ontario. Observations at the Oshawa “coastal chain” taken during the International Field Year on the Great Lakes are then scrutinized, concentrating on three flow-reversal episodes involving deep, barotropic currents. It is shown that the observed phenomena have characteristics very similar to those of the idealized topographic wave. A legitimate interpretation of the observed 12–16 day periodicities in longshore flow within the coastal zone of Lake Ontario is that they are caused by a combination of internal Kelvin waves and topographic waves. Although the speeds of these two kinds of vorticity waves are very similar in Lake Ontario, a careful consideration of the evidence clearly shows phase separation, particularly so late in the season when reduced density differences lead to a slowing down of internal Kelvin waves.