Fish ladder

A fish ladder, also known as a fishway, fish pass, fish steps or fish cannon, is a waterway structure on or around artificial and natural barriers (such as dams, locks and waterfalls) that provides aquatic animals (particularly fish) a "detour" to facilitate natural movements and migrations of diadromous and potamodromous species.

Fishways enable fish to pass around the water level difference around barriers by swimming and/or leaping up a series of relatively low stepped spillway (hence the term ladder), which is built in the form of a small, serpentine, low-discharge canal to the side of the barrier. Each "step" is created by a low weir and each section between the steps functions as a stream pool, allowing fish to rest temporarily and make graduated ascent/descent until they finally reach the main body of water on the other side of the barrier. The velocity of water falling over the step 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.