St Lawrence Seaway
Locks are necessary for vessels to transit the route as the Great Lakes are hundreds of meters above sea level. Locks on the current route were completed in 1959. Along the course of the St Lawrence River there are individual locks. A series of eight locks allow vessels to cross the Niagara Escarpment along the Welland Canal, which connects Lake Ontario and Lake Erie. No locks are necessary to transit from Lake Erie to Lake Huron, or from Lake Huron to Lake Michigan.
All the locks in the system are the same size, except for the locks on the St Mary's River that connect Lake Huron to Lake Superior. The standard locksize is 750 ft (228.6 m) by 80 ft (24.38 m) by 28 ft (8.53 m). This makes the maximum length of vessels 730 ft (222.5 m) because the lock needs to allow room for the lock doors to open. This size of vessel is called seawaymax, just as the size of the largest vessel to be able to transit the Panama Canal are called panamax.
On the St Mary's River there are four parallel locks of differing sizes. The largest lock allows vessels both deeper, wider and longer than those that can transit the rest of the Seaway. Approximately one dozen freighters can use this lock but are too large to travel anywhere but the upper four lakes.