St. lawrence seaway has been called canada's highway to the sea because

As it connects the ocean by a passage of locks and is non continuous  channel.

Explanation:

As about 45$ of trade activities place through this water way. and is able to generate about 238,000 jobs on Canada and the U.S, and is opened since 1959, and flows from the Lake Ontario to the Atlantic ocean. It generally advertised as the highway of water and is under Canada's jurisdiction. Has a 15 locking system and was the first proposal mad for the hydropower projects in the '80s.

Because of the volume of goods that travel to its length

Asmall closed hole formed in an outwash plain or moraine. forms when a  glacier retreats then remains stationary.