To cross a stream, street or valley (in which the delay attributable to a flight of locks at either facet could well be unacceptable) the valley is often spanned by a navigable aqueduct – a popular instance in Wales will be the Pontcysyllte Aqueduct (now a UNESCO Planet Heritage https://tysontchlp.blogdiloz.com/35969899/detailed-notes-on-musica-infantil