To cross a stream, road or valley (where the hold off because of a flight of locks at both aspect will be unacceptable) the valley can be spanned by a navigable aqueduct – a renowned instance in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Environment Heritage Website) https://canalinfantil55543.ageeksblog.com/35831620/canal-infantil-fundamentals-explained