Eli5 How do canal locks work?


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?

In: 5

If you are a barge coming through and you want to go up the lock, you adjust the water (by opening the lower gate) so the lock is empty, drive in, shut the gate and let water in to raise the level then open the top gate and drive out.

You do this in reverse to go down a lock.

The days you see them full someone has gone up through them recently, the days they are low someone has gone down them

[This](https://youtu.be/SBvclVcesEE) video does a fantastic job going over the workings of canal locks. To sum it up, you store water in one place, and let gravity and pumps use that water to lower or raise sections between locks. The whole point is that the water levels on either side of the canal are naturally uneven, so a direct water path would flood. So if you come in from the high side, they take you in at high level, close you off from the natural water, lower you down to the low level, then let you go on your way.

The water on both sides of the lock are at completely different levels. The two gates are NEVER open at the same time.

When a boat approaches, the one gate opens to let it in the middle of the the lock. Once that gate is closed then the gate in front opens to let the water in or out to raise or lower the water under the boat (depending if it is going upstream or downstream).

Then the boat can go on at the different level.

Yes, people are adjusting them – depending on whether they want to go up or down.

If the lock is full of water it means the last boat to go through went up. If it is empty the last boat to go through went down.

Think of it like a normal lift – just one that doesn’t get used much. Sometimes it will be at the top (if someone just came up), sometimes at the bottom (if someone just went down), occasionally moving between them.

If you want to use the lock, if the water (like the lift car) is already at your level you just go into the lock, close the gates behind you so nothing – including water – can get in or out that end (like closing the lift doors), and then you open the small water gates in the other end so water can flow either out of the lock into the lower part of the canal (if you are going down), or into the lock from the upper part of the canal (if you are going up). The water level in the lock will eventually reach the same level as whatever part of the canal it is “open” to (so the bottom if you are going down, the top if you are going up) as water either flows into or out of the lock. Once you get to the level you are at you can open the main gates at that level and move off (ideally closing the small water gates behind you, so the next person doesn’t have to).

If the water isn’t already at the level you want when you get there, you have to cycle the lock your way first (like calling a lift car to your level); so bring the water in the lock to your level, go in, bring the water in the lock to the other level.

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.

I was once allowed to pull the levers for a lock on the Erie Canal. Some old houseboat was passing through.

There’s usually someone on shore who will open the gates as needed via a lever or switch. If the lock is full, the operator can accept a boat from uphill by opening that gate, closing it once the boat is in, and opening the downhill gate to drain the lock. It looks like draining a bathtub. If a lock is empty, the operator can do the same in reverse for a boat on the downhill side. Most of the time, it’s better to leave the lock as-is after a boat passes through. Why waste the water draining or filling it until you know you need to?

In a very small canal or a very remote location, I could imagine a boat operator being forced to tie up their boat and operate a lock themselves.