To cross a stream, road or valley (where by the hold off because of a flight of locks at both facet would be unacceptable) the valley could be spanned by a navigable aqueduct – a well known illustration in Wales would be the Pontcysyllte Aqueduct (now a UNESCO Earth https://jaredunuyb.activablog.com/36286859/helping-the-others-realize-the-advantages-of-musica-infantil