To cross a stream, road or valley (where the hold off a result of a flight of locks at both facet might be unacceptable) the valley is often spanned by a navigable aqueduct – a well known illustration in Wales would be the Pontcysyllte Aqueduct (now a UNESCO Planet https://canalinfantil11109.worldblogged.com/43324333/the-2-minute-rule-for-urso-fofo