Fish ladder - A fish ladder is made up of a series of stepped pools that link the river downstream of a dam to the reservoir behind it. Water flows continuously down the ladder during the migratory season for the fish that use the river. The level of the water between each pool varies by about 300-600 mm. The pools are separated by small weirs that allow water to flow over them, or they are linked by short sloping underwater pipes. Sometimes both weirs and pipes are used.
Cross-section of a 'pool & weir'
Cross-section of a 'weir & orifice' fish ladder