To cross a stream, road or valley (the place the hold off due to a flight of locks at either aspect could be unacceptable) the valley may be spanned by a navigable aqueduct – a renowned example in Wales will be the Pontcysyllte Aqueduct (now a UNESCO Earth Heritage https://paxtonnidvo.blogpixi.com/37351789/a-secret-weapon-for-musica-infantil