To cross a stream, road or valley (where the hold off because of a flight of locks at both aspect will be unacceptable) the valley may be spanned by a navigable aqueduct – a renowned instance in Wales will be the Pontcysyllte Aqueduct (now a UNESCO World Heritage Web https://andersonsycim.blogpostie.com/58349695/not-known-details-about-urso-fofo