To cross a stream, highway or valley (where by the hold off a result of a flight of locks at both side will be unacceptable) the valley is usually spanned by a navigable aqueduct – a well-known example in Wales is the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage https://claytonatlwr.blogdigy.com/the-best-side-of-urso-fofo-56922232