To cross a stream, highway or valley (where by the hold off due to a flight of locks at both aspect might be unacceptable) the valley can be spanned by a navigable aqueduct – a famed instance in Wales is the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage Web-site) https://urso-fofo67765.blogtov.com/17655354/urso-fofo-can-be-fun-for-anyone