Locks use the law of gravity to raise and lower the water level. No pumps are used except to pump out debris from behind the gates. As you can see in the animation, valves are opened to allow water to rush in and equalize itself with it's upper level, gates are opened and the tow pulls into the chamber. The upper gates are closed and the valves are turned off and the other valves are opened to allow the water to equalize itself with the lower level. Then the lower gates are opened and the tow goes on it's merry way.

This graphic and information were copied with permission from Towboat Joe's previous web site which no longer exists.