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Locks allow canal boats to navigate sloping terrains by raising or lowering the level of the water. To move upstream a boat enters the lock through the lower gate, then the gate is closed and the boat is trapped in a stretch of water between the lower and upper gates. Paddles are opened in the upper gate to slowly feed water into the pound, and this raises the surface of the water until it is level with the water upstream.

While there remains any difference in surface level between the water inside the pound and the water upstream, the upper gate is impossible to open because of the accumulated pressure on the upstream side.


The very good animation above is from http://www.haworth-village.org.uk.