Why are ocean vessels able to travel to Ontario's main cities?

Ocean vessels can travel to Ontario's main cities, such as Toronto, due to the St. Lawrence Seaway, which provides a navigable route from the Atlantic Ocean to the Great Lakes. This system of locks and channels allows large ships to bypass elevation changes and access inland ports. Additionally, the Great Lakes serve as a significant transportation network, facilitating the movement of goods and materials to and from the region. However, the size of vessels that can navigate this system is limited by the dimensions of the locks and channels.