To cross a stream, highway or valley (where the delay caused by a flight of locks at either side might be unacceptable) the valley may be spanned by a navigable aqueduct – a renowned instance in Wales may be the Pontcysyllte Aqueduct (now a UNESCO Entire world Heritage Web https://musicainfantil99876.tokka-blog.com/37439811/canal-infantil-for-dummies