To cross a stream, road or valley (wherever the hold off due to a flight of locks at either side could be unacceptable) the valley may be spanned by a navigable aqueduct – a well-known example in Wales may be the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage Website) https://andyszfls.mybuzzblog.com/17000362/5-easy-facts-about-musica-infantil-described