Fish passes in river systems

Description of measure

This measure is concerned with a range of in-channel structures, typically referred to as fish passes, designed to facilitate the upstream and downstream movement of fish and other aquatic fauna.  Fish passes can be applied to watercourses where natural or human placed obstructions such as dams, weirs, or culverts prevent or interfere with fish migration.  This measure does not cover Bypass channels or Mitigation for pumping station intakes, which are covered elsewhere in this Manual as separate measures.


Fish passage problems can occur at almost any site where the water level difference between upstream and downstream of the structure is greater than about 0.5m (Environment Agency, 2009). Typically these sites can be identified by fish leaping clear of the water in an attempt to ascend the structure. Where fish passage is adequate fish do not usually leap.

Passage projects are more likely to provide long-term benefits in channels that are vertically and laterally stable. In less stable channels, passage structures or features may become buried (in aggrading channels), undermined (in degrading channels) or abandoned (in laterally migrating channels) (Saldi-Caromile, 2004).

Where a fish pass is being considered, the aquatic ecosystem should be carefully evaluated to assure that the pass would not adversely impact other aquatic biota and stream corridor functions. In some watercourses, obstructions act as barriers to the movement of undesirable species, such as Topmouth Gudgeon. The installation of a fish pass can encourage the spread of such species to previously isolated sections of a watercourse.

Benefits for the Water Framework Directive

The free passage of migratory fish is a key requirement of the Water Framework Directive, and is being used as an indicator for assessing whether water bodies are meeting Good Ecological Potential or Status. Initial assessments suggest that many waters throughout the UK are at risk of failing to achieve Good Ecological Potential as a result of barriers to fish migration. Well designed fish passes can help deliver objectives of the Water Framework Directive, by:

  • Ensuring that fish can move freely between the river and coastal waters in order to access breeding, nursery of feeding grounds.
  • Allowing passage of other mobile aquatic species, such as invertebrates and plankton.

Fish passes can provide a range of other benefits in addition to those associated to fish passage. Certain types of fish pass can also contribute to longitudinal sediment transport. They can also assist with nutrient transport and oxygenation if the fish pass is associated to an impoundment structure. Where fish passes are installed with interpretive material and public displays, they can also play a role in awareness raising and educating local stakeholders.

To read more about the effectiveness of the measure within academic literature please click here: Effectiveness for Biological Quality Elements

Types of Fish Pass

Fish pass designs vary in form, function and complexity depending on the site and the target species. For the purposes of this Manual, fish passes have been divided into six broad categories:

  • Pool and weir passes
  • Baffled passes
  • Fish locks
  • Pre-barrages
  • Rock ramp passes
  • Bypass channels



Description / Application

Pool and weir passes


This type of pass consists of a number of pools, arranged in series of steps separated by cross walls. These pools dissipate the energy of the falling water and providing resting areas for ascending fish. The pools need to be large enough and deep enough to keep turbulence to a minimum and the velocity of the water through the steps must be within the swimming speed of the target species of fish. Typically the upstream gradients achievable with these passes are of the order of 10-15 per cent (Environment Agency, 2009).

Pool & Weir fish pass Boroughbridge, River Ouse
Source: Armstrong et al. (2004)

Variations on pool and weir passes include pool and traverse passes, vertical slot and orifice passes. Many coarse fish species prefer pool and orifice or vertical slot passes rather than pool and traverse passes, because of their preference for swimming close to the bottom.

Example of a slot pass with two slots
Source: FAO/DVWK (2002)             

Rock ramp passes

The rock ramp whole weir replacement, sometimes referred to as ‘Bottom ramp’ or ‘Slope ramp’, is integrated directly into the weir and extends across the entire width of the weir. All the discharge therefore falls down the ramp. This type of rock ramp is only really suitable for weirs where upstream water levels do not need to be tightly controlled and a flow control system is not required, e.g. old weir structures, weirs used for channel stabilisation or those simply used to maintain a headwater level.

Rock ramps may also be integrated into the weir to form partial weir replacements, where the ramp only extends partially across the weir.

Adapted from - FAO/DVWK (2002)

Baffled passes

This type of pass generally uses a sloping rectangular channel with a series of precisely positioned and shaped plates or 'baffles'. These baffles redirect the water flowing down the fish pass channel, dissipating power and reducing the average water velocity. These fish passes can operate at channel gradients of 15 or even 25 per cent (Environment Agency, 2009). There are four main types of baffle passes:

Larinier Pass – has no side baffles, but is fitted with baffles on the bed of the pass, and is generally wider than the other types (baffles on the floor of the pass only). The Larinier pass is becoming more commonly used in England and Wales due of the range of species that can use it, lower maintenance (lower chance of blockages), flexibility in width, and the fact that it generally has a good flow attraction.

Larinier bottom baffle pass River Neath, Aberdulais (Source: Armstrong et al., 2004)

Plane Baffle Pass (Denil) – contains a series of planar baffles, on walls and floor, pointing upstream at an angle of 30-40 degrees to the flow (baffles on floor and walls).


Double Denil pass, Andrews Stream Barcombe Mills, River Ouse, Sussex (Source: Royal Haskoning, 2009)

Alaskan – narrow pass that allows for steep slopes (baffles on floor and walls), and was originally designed for Pacific salmon.

They are very suitable for migratory salmonids and trout.  The relatively low velocities also make these passes useful for larger coarse fish species, provided that gradients ≤20% are used. They have the disadvantage of blocking and the relatively limited discharge that reduces attraction for fish species.

Source: Armstrong et al., 2004

Chevron – baffle passes that allow for the passage of fish and small craft (baffles on the floor of the pass only).  The Chevron type pass is not described further in this section because it is less hydraulically efficient than the other pass types and is generally only suitable for large salmon.

Fish locks

A fish lock operates on the same principle as a navigation lock in order to secure the passage of fish. They usually consist of a sloping or vertical cylinder constructed as an integral part of the dam structure (or in a few instances as a later addition to the structure), which connects two pools, one located at the upstream level and one at the downstream level. The pools are each provided with a sluice gate that controls the operation of the structure. In contrast, fish lifts operate on the principal of a conventional elevator (fish lifts are not considered any further here).

Fish locks are used where the obstruction excludes the use of other types of fish pass. In particular, hydro-power dams can form obstructions many tens of metres in height. At gradients of 10-25% the size of the fish pass can become very large and therefore make it difficult to place the entrance of the fish pass close to the natural gatherings of ascending fish.


Fish passage at small obstructions, in terms of the vertical height which has to be traversed, can be helped by provision of a small weir or weirs downstream of the main obstruction. These have the effect of splitting the distance to be traversed into smaller leaps or traverses.

Pre-barrage Glaxo Newland, Newlands Beck

Source: Armstrong et al. (2004)

Bypass Channels

A bypass channel is a type of fish pass which wholly circumvents the barrier and where possible resembles, in form and function, a side channel or natural tributary of the main river system. This type of fish pass is considered as a separate measure within this Manual (please see: Bypass channels)


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