To cross a stream, road or valley (exactly where the delay caused by a flight of locks at either aspect could be unacceptable) the valley may be spanned by a navigable aqueduct – a well-known illustration in Wales would be the Pontcysyllte Aqueduct (now a UNESCO World Heritage Web-site) https://johnnydjpwa.blogolize.com/the-definitive-guide-to-urso-fofo-76138302