Locks 4, 5, and 6 are twin locks in the City of Thorold that overcome the steep rise of the Niagara Escarpment.
This set of locks function like giant steps in a large flight of stairs. At this point, the locks will raise or lower a ship three times the distance as a regular lock, since this is where the Niagara Escarpment falls. It also allows for two-way traffic, so a vessel can be going upbound, at the same time that one is going downbound. The water levels always have to be correct, so the vessels can traverse the escarpment smoothly. It truly is an engineering masterpiece!