To cross a stream, road or valley (wherever the hold off brought on by a flight of locks at both side would be unacceptable) the valley is often spanned by a navigable aqueduct – a famous illustration in Wales may be the Pontcysyllte Aqueduct (now a UNESCO World Heritage https://felixqyehm.blazingblog.com/37096023/detailed-notes-on-musica-infantil