Fish ladder
A channel of steps or pools built beside a dam or weir so that migrating fish can swim past it.
Also: fish pass, fishway
A fish ladder is a series of low steps, pools or baffles that let fish climb past a dam a little at a time. Salmon ladders on rivers such as the Columbia in the United States are well known, and some dams count each fish through a window. Ladders work best for strong swimmers; many species, including hilsa and smaller carps, use them poorly. Other designs include fish lifts, which carry fish up in a tank, and nature-like bypass channels.