To cross a stream, highway or valley (in which the delay due to a flight of locks at possibly side will be unacceptable) the valley may be spanned by a navigable aqueduct – a renowned illustration in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Earth Heritage Internet https://canalinfantil67776.blogzet.com/considerations-to-know-about-canal-infantil-51892047