To cross a stream, road or valley (exactly where the hold off because of a flight of locks at both aspect can be unacceptable) the valley might be spanned by a navigable aqueduct – a renowned example in Wales is the Pontcysyllte Aqueduct (now a UNESCO Environment Heritage Internet https://musica-infantil23322.bloggazza.com/36067864/5-essential-elements-for-musica-infantil