To cross a stream, highway or valley (where by the delay brought on by a flight of locks at either facet might be unacceptable) the valley is often spanned by a navigable aqueduct – a popular illustration in Wales would be the Pontcysyllte Aqueduct (now a UNESCO Planet Heritage https://elliottiwsbj.bloggactivo.com/36320154/5-easy-facts-about-musica-infantil-described