Lock (water transport)
A 'lock' is a place where boats that are travelling up or down a river or canal can be raised higher or lower. Locks are built in places where the level of the water in the river or canal suddenly changes. This may be because there is a waterfall there, or because a dam or a weir has been built, or because there is some other thing in the way. The lock is like a big chamber with gates at each end. They have lock gears which empty or fill the chamber with water.