The locks referred to here are the elevators that are used to transport boats safely from one water level to another in dams. These two varying water depths allow river traffic to operate The attached picture shows how boats enter locks in dam sites.
To regulate traffic, there are traffic lights that signal boatmen to adjust their speed when approaching the lock. The red light means to stop and to steer clear away from the lock to allows the boats inside to exit. The green light signals to enter the lock. Lastly, the amber light means approach the lock at a safe speed and under full control.
