A
fish ladder, also known as a
fishway,
fish pass or
fish steps, is a structure on or around artificial and natural barriers (such as
dams,
locks and
waterfalls) to facilitate
diadromous fishes' natural
migration. Most fishways enable fish to pass around the barriers by swimming and 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.