To cross a stream, highway or valley (where by the hold off a result of a flight of locks at both aspect could be unacceptable) the valley can be spanned by a navigable aqueduct – a famous example in Wales is the Pontcysyllte Aqueduct (now a UNESCO Environment Heritage https://urso-fofo89887.livebloggs.com/43662213/urso-fofo-can-be-fun-for-anyone