To cross a stream, road or valley (where the hold off caused by a flight of locks at both aspect will be unacceptable) the valley may be spanned by a navigable aqueduct – a well-known case in point in Wales will be the Pontcysyllte Aqueduct (now a UNESCO World https://musica-infantil13277.total-blog.com/the-ultimate-guide-to-urso-fofo-62416125