To cross a stream, highway or valley (wherever the delay because of a flight of locks at possibly facet would be unacceptable) the valley could be spanned by a navigable aqueduct – a well known case in point in Wales is definitely the Pontcysyllte Aqueduct (now a UNESCO Globe https://ursofofo12210.rimmablog.com/36109179/not-known-facts-about-canal-infantil