To cross a stream, road or valley (where the delay caused by a flight of locks at either side could be unacceptable) the valley is often spanned by a navigable aqueduct – a well known case in point in Wales is the Pontcysyllte Aqueduct (now a UNESCO Earth Heritage https://urso-fofo80099.ka-blogs.com/90277326/the-ultimate-guide-to-urso-fofo