To cross a stream, road or valley (where by the delay because of a flight of locks at possibly side will be unacceptable) the valley is often spanned by a navigable aqueduct – a famous case in point in Wales will be the Pontcysyllte Aqueduct (now a UNESCO World https://dallastcmbs.mybloglicious.com/57119188/the-definitive-guide-to-canal-infantil