Lock (water transport)

A lock is a part of a navigable waterway system that makes a water "channel" deep enough for vessels to use. The lock controls pool depths, for example in a lock and dam system across a waterway. 
The lock is a place where boats that travel up or down a river or canal can be moved to the next higher or lower level. Locks are built in places where the level of the water in the river or canal suddenly changes. This may be because of a waterfall there, or because a dam or a weir has been built, or because some other thing is 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. Locks help a river to be more easily navigable (easier for boats to travel up and down), or for canals to be built across country that is not level.
How a lock works.
If a boat that is travelling downstream (in the same direction that the water is flowing) arrives at a lock, this is what happens:
If a boat that is travelling upstream (in the opposite direction to the water flow), the opposite happens:
The whole process of going through a lock may take about 15 to 20 minutes, depending on whether the boat has to wait. Some locks can take several boats at once, and the first one to enter may have to wait until other boats arrive.