The operation of a lock is very similar to filling or emptying a bathtub. It is based on the fact that gravity allows water to flow downhill. Valves are located at each end of the lock to fill and empty the lock chamber. A tow going upstream will enter the lock and the lower gate will close. The emptying valves are then closed and the filling valves are opened.
Gravity allows the water to rush into the lock, raising the water level in the lock chamber.
When the water levels are equal, the upper gates are opened. When a tow travels downstream, the process is reversed. The main lock can hold nine barges at a time. The maximum number of loaded barges that a towboat can push on the Upper Mississippi is 15. When a towboat has to separate the load, called a double lockage, a cable is used to pull the barges out when heading upstream and the second half comes in with the towboat.