fish ladder

kalaporras, kalatie A structure built on or around dams or locks to facilitate the migration of fish.
1941, (w), (w), Chapter 18, http://www.gutenberg.net.au/ebooks01/0100131h.html

We came to the base of an abnormally long perpendicular fish ladder, stretching up, up into shadow so overwhelmingly deep it seemed as if a pit had been inverted over our heads.