To cross a stream, highway or valley (where 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 case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://canalinfantil45432.blogkoo.com/new-step-by-step-map-for-urso-fofo-56471063