the narrow channels are impassable to oceangoing ships
incapable of being passed
Impassibility (from Latin in-, "not", passibilis, "able to suffer, experience emotion") describes the theological doctrine that God does not experience pain or pleasure from the actions of another being. ...
Snow cover thick enough to make road impassable for normal vehicles. This description is also valid when a road is impassable for other reasons, such as landslides, water floods etc.