To cross a stream, highway or valley (wherever the hold off due to a flight of locks at both aspect could be unacceptable) the valley may be spanned by a navigable aqueduct – a famous case in point in Wales is the Pontcysyllte Aqueduct (now a UNESCO World Heritage https://urso-fofo33220.topbloghub.com/43493035/the-best-side-of-urso-fofo