To cross a stream, road or valley (in which the delay caused by a flight of locks at both aspect would be unacceptable) the valley can be spanned by a navigable aqueduct – a well known illustration in Wales will be the Pontcysyllte Aqueduct (now a UNESCO Earth Heritage https://sergiopmhdw.ourcodeblog.com/37397353/new-step-by-step-map-for-urso-fofo