Which waterway was built to help protect Canada in case of an invasion from the USA?

The Rideau Canal was constructed in the early 19th century with the primary purpose of providing a secure and navigable route between Montreal and Lake Ontario, effectively bypassing the vulnerable stretch of the St. Lawrence River. The canal was strategically designed to facilitate military movement and resupply in the event of a potential invasion from the United States, particularly during a period of heightened tensions following the War of 1812.

Its path takes advantage of the natural terrain, utilizing a combination of lakes and rivers connected by artificial locks, which made it not only a military advantage but also a key route for trade and travel. The intention behind the canal was to enhance the defense system of what was then British North America by ensuring that supplies and military forces could move freely between critical areas without reliance on potentially hostile waters.

Other waterways listed serve different purposes: the Erie Canal was a significant transportation route for commerce, the Soo Locks facilitate the passage of ships between Lake Superior and the lower Great Lakes, and the St. Lawrence Seaway is primarily a commercial shipping route. However, none of these were specifically created with the intent of bolstering national defense against a potential invasion in the way that the Rideau Canal was designed.