To cross a stream, highway or valley (where by the hold off due to a flight of locks at possibly facet could be unacceptable) the valley can be spanned by a navigable aqueduct – a famous illustration in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Earth Heritage https://canal-infantil22210.theobloggers.com/43541760/a-review-of-canal-infantil