Leonardo da Vinci improved the canal lock by using two mitre gates to seal a chamber.
A mitre lock has two gates that meet at an angle pointing upstream. Water pressure pushes the gates together, helping them resist the force of the water. This arrangement allows a boat to move between water levels: the chamber is filled or emptied after the gates are closed in the appropriate sequence.
Leonardo worked on canal engineering in Milan, where waterways were important for transport and construction. His lock designs are associated especially with the San Marco basin and the navigation system connecting Milan with nearby rivers and lakes. The lock concept itself was not invented entirely by Leonardo, but he refined and applied hydraulic ideas in practical engineering projects.
A canal lock is different from a sluice gate, which controls water flow but does not normally raise or lower boats in a chamber. Leonardo’s contribution belongs to the history of improving navigation infrastructure rather than creating a wholly new machine.