To cross a stream, street or valley (in which the delay a result of a flight of locks at either facet would be unacceptable) the valley may be spanned by a navigable aqueduct – a well-known example in Wales is definitely the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage https://laneekotv.blogsidea.com/43675150/a-review-of-canal-infantil