To cross a stream, street or valley (the place the delay caused by a flight of locks at both side could be unacceptable) the valley could be spanned by a navigable aqueduct – a famous case in point in Wales will be the Pontcysyllte Aqueduct (now a UNESCO World https://canalinfantil11198.bloggazza.com/36064039/the-definitive-guide-to-canal-infantil