The St. Lawrence is Tidal all the way inland to the lower part of Quebec City (before the gorge), something I always thought remarkable since Quebec City is 600 miles inland from the Atlantic. (officially 946 miles inland from the Atlantic but for ocean freighters Quebec City 600 miles inland is what they use because they don't count the distance from the Bay to the Ocean as being "inland")

Theoretically even though there would be LOTS of land to flood and distance to cover as an tsunami came in from the Atlantic you could get a several foot rise in the water by the docks in lower Quebec City (most of the city itself is up on the cliffs above), but only a few feet imo.

Because of the distances involved, and the gorge at Quebec City that the St. Lawrence goes through, it is unlikely imo that a LaPalma tsunami could reach up the St. Lawrence as far as Montreal, let alone going above the Montreal rapids into Lake Ontario.