Left and right in (call it) riverese (and possibly also in other contexts) is relative to your point of view. Now left and right in dictionary definition means that the bank on the left of the direction of flow is left, the bank on the right of the direction of flow is right. The Seine flow roughly east to west through Paris. Therefore the Rive Gauche is to the south, the Rive Droite is to the north. This method of naming is formalized in the geological science of orography, the study of topography of hills and mountains. Thus rivers as seen from their source (which is topographically highest). As seen from the source downward, the banks remain always to left or right of the water.