Cornwall Canal
The Cornwall Canal was one of eight canals that connected western Canada with the ocean by way of the Great Lakes and the St. Lawrence River. Constructed in 1834-1842 to overcome the rapids of the Long Sault, it was enlarged several times between 1876-1904 and superseded by the St. Lawrence Seaway in 1959.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.