To cross a stream, road or valley (exactly where the delay caused by a flight of locks at both side would be unacceptable) the valley can be spanned by a navigable aqueduct – a famed case in point in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Environment https://canalinfantil56655.glifeblog.com/35946360/a-review-of-canal-infantil