To cross a stream, road or valley (exactly where the hold off because of a flight of locks at either side could be unacceptable) the valley may be spanned by a navigable aqueduct – a famous instance in Wales would be the Pontcysyllte Aqueduct (now a UNESCO Planet Heritage https://hectorltxcg.loginblogin.com/44898507/not-known-details-about-urso-fofo