To cross a stream, road or valley (exactly where the delay caused by a flight of locks at either side would be unacceptable) the valley can be spanned by a navigable aqueduct – a famed case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Entire https://edwinschls.blogofoto.com/68304331/helping-the-others-realize-the-advantages-of-musica-infantil