I walk past one everyday on the way to work, one day the middle of the locks is full of water, other days it’s not. How do they work? Is someone adjusting them every day or so? How do boats get through?
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Let’s see if I can do this, as someone who watches locking videos for fun…
So there’s pumps that move the water up the canal if needed to balance out but the water flows down the flights (lock series) via gravity.
The locks themselves are filled or emptied via “gates” which are smaller sliding doors in each larger lock door that allow water in or stop it from entering/exiting.
If a lock is empty and the upper doors/gates closed, the level stays even with the downstream side. A boat comes into the lock (if it’s open and no one is coming the other way) and ties up along one side, the pilot gets out on the side of the towpath or lock chamber, and closes the lower lock doors and gates behind their boat. Either manually or with a crank or via motorized hinges. Depends on the lock system. Or a lock keeper engages a motor to close them if it’s mechanized like maybe in a big river city like London.
The upper doors of the lock remain closed too at this point, but the upper gates are opened to fill the chamber to the level of that stage of the river which is kept there by the upper doors, like a small dam. When the water equalizes and is floating at the level of the upstream river, the upper lock doors open and the boat putters on upstream or to the next lock flight.
The water level usually is just kept at whatever recent state it was used for. It takes too much energy to move that water volume mechanically and time to cycle the lock. So they wait until a boat comes downstream, let them in the lock the last boat left, and then drain the water back down via the lower door gates to match the downstream level. If they just opened the doors, the boats and water would crash down and break a lot.
They try to move as many boats as will fit in larger locks at once, to reduce the number of locking cycles they have to do. If a boat missed the upward movement, oftentimes they have to wait until another cycle is scheduled to go upstream because the lock is already filled with water waiting to let boats back downstream.
If the lock is all manually operated, alone by the boater, then you have to pay attention as you come up to it. If it’s closed, you wait. If it’s empty, you can start cycling and wave a lot to anyone coming the other way so they know to wait.
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