To cross a stream, road or valley (where the hold off a result of a flight of locks at both facet could well be unacceptable) the valley may be spanned by a navigable aqueduct – a well-known example in Wales is the Pontcysyllte Aqueduct (now a UNESCO Environment Heritage https://canal-infantil99876.dbblog.net/10326257/rumored-buzz-on-canal-infantil