To cross a stream, road or valley (where by the delay because of a flight of locks at both aspect could be unacceptable) the valley may be spanned by a navigable aqueduct – a well known case in point in Wales will be the Pontcysyllte Aqueduct (now a UNESCO https://canalinfantil18506.blogdosaga.com/37013184/what-does-canal-infantil-mean