To cross a stream, highway or valley (where the hold off caused by a flight of locks at possibly side could be unacceptable) the valley may be spanned by a navigable aqueduct – a famous case in point in Wales is definitely the Pontcysyllte Aqueduct (now a UNESCO Environment https://urso-fofo90000.canariblogs.com/urso-fofo-things-to-know-before-you-buy-51955756