To cross a stream, road or valley (in which the delay brought on by a flight of locks at possibly facet might be unacceptable) the valley might be spanned by a navigable aqueduct – a renowned example in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage https://claytonatlwr.blogdigy.com/the-best-side-of-urso-fofo-56922232