The Environment Agency provides guidance for run-of-river hydropower schemes in England, including any potential impacts on how fish are able to move upstream and downstream.

Although the impact of an individual small-scale scheme could be low or potentially positive, concerns about the cumulative impacts of multiple schemes on a single river or within a river catchment led to a project to determine how these can be assessed.

What we did

We developed a model to assess the impact of multiple hydropower schemes on fish movement and migration.

Small scale hydropower schemes are required to include mitigating measures such as a fish pass and can have a positive, neutral or negative effect depending on the design and location of the scheme.

Whether multiple schemes within a catchment have an overall negative, neutral or positive effect therefore depends on the net effects of each individual scheme, considered collectively.

The model forecasts these effects at a catchment scale using three elements: hydropower scheme design and potential impacts, spatial variation in fish populations and changes in the fish life-cycle.

In terms of the hydropower scheme, this includes effects that are important to migratory fish and those that can be quantified. These are:

1) Impediment to upstream and downstream migration

2) Alleviation of upstream and downstream impediment (e.g. installation of a fish pass)

3) Impingement and entrainment

4) Habitat loss to freshwater stages (via a depleted reach)

The model can be modified to include other effects if new research provides better quantification, for example on migration delay caused by barriers.

The model’s spatial population element was developed by applying reference juvenile density values to the wetted area of the river network, using Atlantic salmon as a demonstration species.

The hydropower scheme effects element of the model was then applied to the spatial population element. This enabled the salmon population upstream of a scheme and returning adult salmon to be quantified. Any benefits or dis-benefits of hydropower schemes could then be applied to the model.

By sequentially evaluating multiple schemes in this way we were able to use the model to assess their cumulative effects.

The development of the model is available as an Environment Agency Evidence Report.

Bypass channel for fish around a small scale hydropower scheme

Archimedes-screw type small scale hydropower scheme with a nature-like bypass channel (left) for fish.

BG Shape 5 - Wave


  • A number of hypothetical scenarios confirmed that cumulative effects on fish movement and migration were possible, whether positive, neutral or negative
  • The extent of these effects depended on the location of the hydropower scheme, the combined net benefit or dis-benefit of all schemes, and the status of the fish population
  • The model illustrates the impact of individual or multiple hydropower schemes, both actual and hypothetical, including the effects of different options at a specific site. Thus potential or catchment-scale cumulative effects can be forecast, which may assist in the strategic planning of hydropower schemes
  • Additionally, the model could also be applied to impacts that are not related to hydropower schemes, such as the removal of existing barriers to fish migration or river restoration techniques
Bypass channel for fish around a weir as part of a small-scale hydropower scheme