To cross a stream, street or valley (where the delay because of a flight of locks at both side could well be unacceptable) the valley could be spanned by a navigable aqueduct – a well known example in Wales would be the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage https://beckettktycf.blogcudinti.com/37297015/not-known-details-about-urso-fofo