To cross a stream, highway or valley (wherever the hold off a result of a flight of locks at both aspect could be unacceptable) the valley can be spanned by a navigable aqueduct – a famous case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://connerirxbf.theobloggers.com/43548163/facts-about-urso-fofo-revealed