fishway

A fish ladder, also known as a fishway, is a structure on or around artificial and natural barriers (such as dams, locks & waterfalls) to facilitate fishes' natural migration.

This fish ladder is on the Afon Lwyd (Grey River), next to the weir beneath the aqueduct carrying the Monmouthshire & Brecon Canal (at Pontymoile).

Most fishways enable fish to pass around the barriers by swimming & leaping up a series of relatively low steps (hence the term ladder) into the waters on the other side. The velocity of water falling over the steps has to be great enough to attract the fish to the ladder, but it cannot be so great that it washes fish back downstream or exhausts them to the point of inability to continue their journey upriver.