To cross a stream, highway or valley (wherever the hold off caused by a flight of locks at either facet could well be unacceptable) the valley can be spanned by a navigable aqueduct – a famous case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://simonlswyc.blogrenanda.com/43691262/a-secret-weapon-for-musica-infantil