Leonardo da Vinci improved canal engineering by studying the mitre lock, whose angled gates meet in a V-shaped pair.
A canal lock raises or lowers boats between water levels. The chamber is closed with gates, and water is admitted or released until the boat reaches the next level. In a mitre lock, two gates meet at an angle pointing toward the higher water. Water pressure helps press the gates together, making the closure more secure.
Leonardo worked on hydraulic engineering in Milan, where canals were important for transport, irrigation, and construction. His studies of lock gates and canal systems helped develop practical ways to manage differences in water level. The exact history of individual lock inventions is complex, so it is more accurate to describe Leonardo as an important designer and improver rather than the sole inventor of every canal-lock principle.
A mitre lock is not a ship lift, which raises a vessel mechanically in a tank. It is also not merely a sluice gate: the lock uses a chamber and paired gates to move boats between elevations.