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.
Written reports of rough fishways date to 17th-century France, where bundles of branches were used to create steps in steep channels to bypass obstructions. A version was patented in 1837 by Richard McFarlan of Bathurst, New Brunswick, Canada, who designed a fishway to bypass a dam at his water-powered lumber mill.  In 1852–1854, the Ballisodare Fish Pass was built in County Sligo in Ireland to draw salmon into a river that had not supported a fishery. In 1880, the first fish ladder was built in Rhode Island, United States, on the Pawtuxet Falls Dam. The ladder was removed in 1924, when the City of Providence replaced the wood dam with a concrete one. Concrete ladders are not always an improvement – the electric field-sensitive organs of the paddlefish are overloaded in the proximity of the rebar and other metal used in concrete construction, preventing them from gaining access to their spawning grounds and contributing to a catastrophic decline in their numbers.[ dubious ][ citation needed]
There are six main types of fishways:
- Pool and weir
- Baffle fishway (Denil, Larinier, Alaskan Steeppass, or other baffle configuration)
- Fish elevator
- Rock-ramp fishway
- Vertical-slot fish passage
- Fish siphon
A pool and weir is one of the oldest styles of fish ladders. It uses a series of small dams and pools of regular length to create a long, sloping channel for fish to travel around the obstruction. The channel acts as a fixed lock to gradually step down the water level; to head upstream, fish must jump over from box to box in the ladder.
A baffle fishway uses a series of symmetrical close-spaced baffles in a channel to redirect the flow of water, allowing fish to swim around the barrier. Baffle fishways need not have resting areas, although pools can be included to provide a resting area or to reduce the velocity of the flow. Such fishways can be built with switchbacks to minimize the space needed for their construction. Baffles come in variety of designs. The original design for a Denil fishway was developed in 1909 by a Belgian scientist, G. Denil; it has since been adjusted and adapted in many ways. The Alaskan Steeppass, for example, is a modular prefabricated Denil-fishway variant originally designed for remote areas of Alaska. Baffles have been installed by Project Maitai in several waterways in Nelson, New Zealand, to improve fish passage as part of general environmental restoration.
A fish elevator or fish lift, as its name implies, breaks with the ladder design by providing a sort of elevator to carry fish over a barrier. It is well suited to tall barriers. With a fish elevator, fish swim into a collection area at the base of the obstruction. When enough fish accumulate in the collection area, they are nudged into a hopper that carries them into a flume that empties into the river above the barrier.
A rock-ramp fishway uses large rocks and timbers to create pools and small falls that mimic natural structures. Because of the length of the channel needed for the ladder, such structures are most appropriate for relatively short barriers. They have a significant advantage in that they can provide fish spawning habitat. 
A vertical-slot fish passage is similar to a pool-and-weir system, except that each "dam" has a narrow slot in it near the channel wall. This allows fish to swim upstream without leaping over an obstacle. Vertical-slot fish passages also tend to handle reasonably well the seasonal fluctuation in water levels on each side of the barrier. Recent studies suggest that navigation locks have a potential to be operated as vertical slot fishways to provide increased access for a range of biota, including poor swimmers.  
A fish siphon allows the pass to be installed parallel to a water course and can be used to link two watercourses. The pass utilises a syphon effect to regulate its flow. This style is particularly favoured to aid flood defence.
Fish ladder in Meuse River the Netherlands
Fish ladder in North Vancouver, Canada
Detail of fish ladder in Uppsala, Sweden
Detail of fish ladder on the River Dart in England.
At the Charles River Dam near Boston, United States
Fish ladders have a mixed record of effectiveness. They vary in effectiveness for different types of species, with one study showing that only three percent of American Shad make it through all the fish ladders on the way to their spawning ground.  Effectiveness depends on the fish species' swimming ability, and how the fish moves up and downstream. A fish passage that is designed to allow fish to pass upstream may not allow passage downstream, for instance.  Fish passages do not always work. In practice a challenge is matching swimming performance data to hydrodynamic measurements.   Swim tests rarely use the same protocol and the output is either a single-point measurement or a bulk velocity. In contrast, physical and numerical modelling of fluid flow (i.e. hydrodynamics) deliver a detailed flow map, with a fine spatial and temporal resolution. Regulatory agencies face a difficult task to match hydrodynamic measurements and swimming performance data.
During the last three decades, the ecological impact of culverts on natural streams and rivers has been recognised. While the culvert discharge capacity derives from hydrological and hydraulic engineering considerations,  this results often in large velocities in the barrel, creating a possible fish passage barrier.
Baffles may be installed along the barrel invert to provide some fish-friendly alternative.    For low discharges, the baffles decrease the flow velocity and increase the water depth to facilitate fish passage. At larger discharges, baffles induce lower local velocities and generate recirculation regions. Unfortunately, baffles can reduce drastically the culvert discharge capacity for a given afflux,  thus increasing substantially the total cost of the culvert structure to achieve the same design discharge and afflux. It is believed that fish-turbulence interplay may facilitate upstream migration, albeit an optimum design must be based upon a careful characterisation of both hydrodynamics and fish kinematics.   
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- To Save the Salmon (1997) US Army Corps of Engineers.
- Fish-friendly waterways and culverts - Integration of hydrodynamics and fish turbulence interplay (2017) The University of Queensland.
|Wikimedia Commons has media related to Fish ladder.|
- A study of the hydraulics of flow over fishways
- Construction of a vertical slot fish passage and eel ladder for the St. Ours Dam ( Richelieu River, Québec)
- Fish Passage Center
- Fish passes. Design, dimensions and monitoring, Food and Agriculture Organization of the United Nations/Deutscher Verband für Wasserwirtschaft und Kulturbau (DVWK), Rome, 2002 ISBN 92-5-104894-0 ISBN 3-89554-027-7 ( Zip download from FTP area of the FAO's EIFAC – European Inland Fisheries Advisory Commission)
- Washington Post, January 31, 2007: U.S. Orders Modification of Klamath River – Dams Removal May Prove More Cost-Effective for allowing the passage of Salmon
- First "Denil" style dam in Illinois.
- Fish Ladders and Elevators not working.
- Upstream fish passage in box culverts: how do fish and turbulence interplay? by Dr Hang Wang and Professor Hubert Chanson, School of Civil Engineering, University of Queensland