Not only did the St. Lawrence Seaway keep the German U-Boats out, the submarines that we built on the Great Lakes couldn’t get to the Atlantic. So, they put them on barges and floated them down the Mississippi.
However, those carriers were very effective in defending our Northern borders. There is no record of any invasion from that direction during World War II.
“Not only did the St. Lawrence Seaway keep the German U-Boats out, the submarines that we built on the Great Lakes couldn’t get to the Atlantic. ...”
The St Lawrence River was never navigable by even the smallest oceangoing vessels, not even in the 18th century.
Locks were necessary, with the earliest getting built in the 1870s. However, they permitted transit of relatively small vessels only, drawing 10 ft or less. And even before, there was the minor obstacle of Niagara Falls, which blocked upriver passage of even the smallest boat.
The Seaway in present form (channels big enough for serious oceangoing vessels, accompanied by hydroelectric plants) was proposed as early as the 1890s, but approval lagged as the governments of Canadian provinces and US states could not reach agreement. Construction did not start until the 1950s, and it opened in 1959.
However, those carriers were very effective in defending our Northern borders. There is no record of any invasion from that direction during World War II.