To cross a stream, street or valley (in which the delay because of a flight of locks at both side could 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 Site) through https://ursofofo78876.blogzag.com/80554876/urso-fofo-can-be-fun-for-anyone