Although it is listed as a port on the map, Decatur AL, located in the Lawrence, AL sector on the Tennessee river at Muscle shoals, apparently cannot be entered by riverine units. The river is not frozen, the area is fully-controlled, but any attempt to move from Tuscumbia to Decatur is rejected by the game. This may be because Muscle Shoals is considered unnavigable from downstream, but that map sector is depicted as navigable.
(Note also that it is Muscle Shoals, not Mussel Shoals. )
-- Mal