To cross a stream, street or valley (in which the delay brought on by a flight of locks at both aspect could well be unacceptable) the valley is usually spanned by a navigable aqueduct – a famed illustration in Wales would be the Pontcysyllte Aqueduct (now a UNESCO World https://megamu43107.eedblog.com/37298528/the-best-side-of-urso-fofo