How a Lock Works
 
Boat Going Downstream
 

Boat enters the lock; gate
closes behind it.
 

Water is released downstream; water level lowers in the lock.
 

When the water level is low enough, the gate opens and the boat leaves the lock.
Boat Going Upstream
 
Boat enters the lock; gate closes behind it.
 
Water enters from upstream; water level rises in the lock.
 
When the water level is high enough, the gate opens and the boat leaves the lock.