To cross a stream, street or valley (in which the hold off due to a flight of locks at both side can be unacceptable) the valley may be spanned by a navigable aqueduct – a renowned instance in Wales may be the Pontcysyllte Aqueduct (now a UNESCO Planet Heritage https://edgarfqyfj.bloggosite.com/44418430/helping-the-others-realize-the-advantages-of-urso-fofo