How does a canal lock raise or lower a boat?

A canal lock changes the water level around a boat so it can move between canal sections at different heights. The boat enters a chamber with gates at both ends. When the chamber is isolated by closed boat gates, separate water passages can change the amount of water inside.

Admitting water from the upper canal raises the chamber surface toward the upper level. The boat floats, so it rises with that surface rather than being lifted directly by a platform. When the levels match, the chamber is at the level of the upper canal.

Releasing water toward the lower canal does the reverse. The chamber surface falls, carrying the floating boat downward until the lower levels match. Water flows through controlled passages. Unequal levels put different water pressures on the two sides of a boat gate, producing a net force on it. Matching levels removes that level difference before the large gate opens; the gate itself is not simply swung open to do the filling.

Once the chamber matches a neighboring canal's level, the corresponding boat gate can provide a passage at that height. The water has done the lifting or lowering while buoyancy keeps the boat at the surface. A boat that cannot climb a hillside under its own power can therefore continue through a canal built at several levels.