On the reverse of the first day cover for the third issue in the "Rivers of Canada" series, Canada Post offers the following description of the St. Lawrence River:

The 1197-km 51. Lawrence River, together with the Great Lakes, forms Canada's most significant waterway, stretching 3781 km into the heart of North America. Once home to the Iroquois nation, it became an important European exploration, settlement and trade route for furs and timber, spurring the growth of Montreal and Quebec City into major commercial centres.

Known to the first nations along its banks as the river of Kanata, the explorer Jacques Cartier named one of its harbours in which he found refuge after the Christian martyr, St. Lawrence. It is by that name this entry point into the heart of the North American continent is known today.

This stamp, issued on August 10, 1993, from the "Rivers of Canada Series" commemorates the Saint
Lawrence River as a route that gave settlers access to the Canadian interior. The design
features a ship on the river and a beluga whale, a species that inhabits the river's estuary.

The map of the Saint Lawrence from the back of Canada Post's first day cover.

Canada Post's first day cover for the third issue in its "Rivers of Canada" series.