To cross a stream, street or valley (wherever the hold off caused by a flight of locks at both aspect could be unacceptable) the valley could be spanned by a navigable aqueduct – a renowned instance in Wales may be the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage Website) https://andersontcgko.mdkblog.com/43099150/considerations-to-know-about-urso-fofo