To cross a stream, highway or valley (the place the hold off caused by a flight of locks at either facet could be unacceptable) the valley can be spanned by a navigable aqueduct – a well-known case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://urso-fofo90000.canariblogs.com/urso-fofo-things-to-know-before-you-buy-51955756