Liftlocks allow for watercraft to navigate from one elevation to another. The following is my understanding of how they work.
They consist of two chambers filled with water and boats can enter which sit on top of huge hydraulic rams. The rams of the two chambers are connected underground allowing for the hydraulic fluid to flow between the two rams.
One boat chamber is normally parked at the top elevation and the other at the bottom of the lift locks.
When a boat, or boats, enter the top chamber the amount of water in that chamber is adjusted to make the chamber heavier than the bottom chamber. The heavier chamber then forces the hydraulic fluid to move from its ram to the lower chamber's ram. The upper chamber will then move downward while pushing the lower chamber upward until each have reached the ends of their travel.
Now the lower chamber is parked at the top of the lift and the upper chamber is parked at the bottom of the lift and the process will begin again when another boat comes along.
The upper chamber does not have to have boats in it to begin the process; a boat can enter the lower chamber and the adjustment of water volume in each chamber will cause the hydraulic fluid to flow and the chambers to move through their cycle. There can be as many boats in either chamber as will physically fit and the lift locks will still work.
I live in a city which, until recently, had the highest liftlocks in the world which were built in 1904.
You can get more information here:
http://mypeterborough.info/liftlocks.php