When it is time for a boat to enter a river lock, a green light flashes. If a red light flashes, boaters are not allowed to proceed. A flashing yellow light informs boaters that they are permitted to move toward the lock at a reasonable speed.
A fixed red light suggests that the lock master has not noticed any boats. In some systems, steady lights are used for green and yellow as well. In other areas, horn blasts indicate the go-ahead for boats. One long signal allows entrance into the main lock, while two long blasts signify that the auxiliary lock is to be used. To exit these locks, corresponding short blasts are sounded.