To cross a stream, highway or valley (where by the hold off due to a flight of locks at both side would be unacceptable) the valley is usually spanned by a navigable aqueduct – a famous case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO https://canalinfantil34432.ourcodeblog.com/37408787/not-known-facts-about-canal-infantil