To cross a stream, street or valley (in which the delay brought on by a flight of locks at either aspect can be unacceptable) the valley could be spanned by a navigable aqueduct – a famed case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://elliottiwsbj.bloggactivo.com/36320154/5-easy-facts-about-musica-infantil-described