To cross a stream, road or valley (the place the hold off a result of a flight of locks at possibly facet might be unacceptable) the valley is often spanned by a navigable aqueduct – a famous case in point in Wales is the Pontcysyllte Aqueduct (now a UNESCO https://zaneweknq.blogpayz.com/37285362/the-ultimate-guide-to-urso-fofo