To cross a stream, road or valley (exactly where the delay caused by a flight of locks at possibly facet could well be unacceptable) the valley is often spanned by a navigable aqueduct – a well known illustration in Wales would be the Pontcysyllte Aqueduct (now a UNESCO Planet https://musicainfantil79988.blogaritma.com/35275048/facts-about-urso-fofo-revealed