To cross a stream, highway or valley (where by the hold off a result of a flight of locks at both side could well be unacceptable) the valley could be spanned by a navigable aqueduct – a popular instance in Wales may be the Pontcysyllte Aqueduct (now a UNESCO https://canal-infantil46543.tblogz.com/helping-the-others-realize-the-advantages-of-urso-fofo-50437355