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How does a canal lock work?

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Quick answer

A canal lock raises or lowers boats between different water levels. The boat enters a chamber, the gates behind it close, and valves let water flow in from the higher side or drain out to the lower side. The boat floats up or down with the water until the level matches, then the gates ahead open.

A canal lock works by moving boats between two water levels inside a watertight chamber with gates at each end. Opening valves lets water flow in from the higher side or out to the lower side, so the boat rises or falls with the water until it matches the level ahead.

Step by step

  1. The lock keeper sets the chamber level to match the boat's side, then opens those gates for it to enter.
  2. The boat moves into the chamber, ties up loosely, and the gates behind it are closed.
  3. To go up, upper paddles open and water flows in; to go down, lower paddles release water out.
  4. The boat rises or falls with the water level, while the crew adjusts ropes to keep it steady.
  5. Once the chamber matches the level ahead, those gates open and the boat continues along the canal.

Key things to understand

  1. A lock is a walled chamber with gates at both ends, placed where a canal or river changes height.
  2. Only one set of gates opens at a time, so water never flows straight through from the upper level to the lower.
  3. Small sluices called paddles let water into the chamber from above or drain it to the level below.
  4. The boat floats on the water in the chamber, so it rises or sinks as the level changes.
  5. When the chamber matches the next level, pressure on both sides of the gates equalises and they can be opened.

Labelled diagram

  1. Lower gates

    Open to let the boat into the chamber at the lower water level.

  2. Chamber

    Watertight basin where the boat floats while the water level changes.

  3. Paddles

    Sluice valves that let water flow in from above or drain below.

  4. Upper gates

    Open once the chamber level matches the upper canal, releasing the boat.

  5. Upper canal

    The higher stretch of water the boat continues along after the lock.

Raising a boat through a canal lock

Real-world examples

  • The Panama Canal's locks lift large ships about 26 metres from sea level up to Gatun Lake and back down.
  • The Farakka barrage on the Ganga includes a navigation lock so cargo vessels can pass the dam.
  • The Caen Hill flight on England's Kennet and Avon Canal climbs a steep hill through 29 locks.
  • Locks on the Rhine, Danube and Saint Lawrence rivers let barges and ships bypass dams and rapids.

Common mistake

✗ Myth: Canal locks use powerful pumps to lift boats uphill.

✓ Fact: Ordinary locks use gravity alone. Water flows from the higher level into the chamber to raise a boat, and drains to the lower level to lower it; only the gates and valves need to be moved.

People also ask

How do canal locks lift boats?
The boat enters a chamber at the lower level, and the gates behind it close. Paddles in the upper gates or side culverts open, letting water from the higher canal pour into the chamber. The boat floats upward as the water rises, and when the levels match, the upper gates open.
Why do canals need locks?
Land is rarely flat, but water in a canal must sit level or it would drain away downhill. Locks divide a canal into level sections at different heights and act as water-filled steps, letting boats climb or descend hills and pass around dams, weirs and rapids on rivers.
Where does the water in a canal lock go?
Each time a lock is emptied, a chamberful of water flows down into the next lower section of the canal. It eventually reaches a river or the sea. Water lost this way is replaced by feeder streams and reservoirs at the canal's highest point, and some locks recycle water through side ponds.
Does a canal lock need pumps?
Most locks need no pumps at all. Water simply flows downhill by gravity from the upper level into the chamber or out to the lower level. Canals do need a supply of water at the summit, often from reservoirs, to replace what each lock operation sends downhill.
Why are lock gates V-shaped?
Many locks use mitre gates, two leaves that meet at an angle pointing upstream. The pressure of the higher water pushes the leaves together, sealing them more tightly, which is why they cannot be opened until the levels equalise.
How long does it take to go through a lock?
A small canal lock might take ten to twenty minutes including entering, filling or emptying and leaving. Large ship locks hold far more water and take longer, and waiting time for other vessels can add much more.
What is a staircase lock?
A staircase lock is a series of chambers built directly against each other, where the upper gate of one chamber is the lower gate of the next. It lets a canal climb a steep slope in a short distance.

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