The St.Lawrence River has three geopolitical functions. As an important gateway of the continent, it always induced those holding the key-positions of Montréal and Québec to build large empires. As a Québec inland sea, it is the basic axis of organization for the linguistic community settled in the St.Lawrence Corridor, and as a big cut in the North American space, it has been chosen as an international boundary, while being an obstacle for all the transcanadian transportation arteries. This paper studies the places of these functions in the present constutional debate, and shows that the St.Lawrence River will play an important part, whatever the option may be.