Beavers in Scotland: consultation on the strategic environmental assessment

A consultation on the policy to reintroduce beavers to Scotland and the strategic environmental assessment of this policy.

4.10 Beavers and running freshwater habitat

4.10.1 How beaver activity affects hydrology, fluvial geomorphology and river habitat Effects on hydrology

Beaver dams will impede the flow (quantity and velocity) of water in a channel. The extent to which they do will depend upon their height and porosity and the frequency at which they occur. Beaver dams therefore increase the in-channel storage of water. When dams extend beyond the channel, floodplain storage will also increase. One study concluded that abandoned beaver ponds played a role in increasing channel retention, and that changes in in-channel storage resulting from beaver dams were a positive aspect of beaver activity. By increasing the amount of water stored in a channel or on a floodplain the effects of prolonged periods of dry weather may be lessened. Some of the findings of a recent literature review include that beaver dams moderate stream flow, increase surface water and riparian groundwater storage, regulate hyporheic flows (i.e. flows in the groundwater- surface water mixing zone, which is now known to be important for the maintenance of running water habitat) and enhance evapotranspiration rates (i.e. the evaporation of water from plants and the earth's surface).

By slowing flow, and therefore reducing the speed at which intercepted precipitation passes through a catchment, beaver dams can increase the length of time taken for a flood to reach its peak and reduce the height of the peak. Beaver activity may therefore result in the development of natural flood defences. Investigations of the effects of dams on flow have been undertaken in North America and Europe. Following the reintroduction of Eurasian beaver to Belgium, one study investigated some of the effects of their dams on hydrology. It indicated a significant lowering of peak flow downstream of dams, an increase in the length of interval between major floods, and an increase in the depth of low flows. Another study in Glacier National Park, Montana, found that North American beaver dams reduced the velocity and quantity of water emerging downstream of them and that older dams had a greater effect than newer ones.

Modifications to stream hydrology as a result of beaver activity are unlikely to be solely in response to dam building. In-channel accumulations of wood are a feature of many naturally functioning river systems, and wood derived from beaver activity is likely to increase both the total amount of material available and the incidence of accumulations. Investigations into the effects of accumulations of coarse wood in streams in the New Forest showed an increase in the amount of time taken for water to pass through a channel.

Localised changes in the connectivity between channels and their riparian zone and floodplains are likely, including alternating patches of high and low water table. Beaver canals may increase channel-floodplain connectivity including via the connection of previously discrete floodplain water bodies with a stream or river. Effects on geomorphology

Beaver dams will not only attenuate flow but also impede the movement of sediment. As the ability or 'competence' of a flow to transport sediment decreases, fine material will begin to fall out of suspension and coarser material will come to a stop. These interruptions to sediment transport will happen upstream of beaver dams where flowing water enters a ponded reach. Work in Glacier National Park showed that beaver ponds clearly trapped sediment and that the depth and volume of sediment substantially increased with dam age. In the Republic of Tatarstan, Russia, three beaver dams stopped 4,250 tonnes of particles in the Sumka River during a period of flooding in 2001.

The dissipation of energy associated with flows slowed by beaver activity will result in increased channel stability, i.e. less erosion and deposition and therefore less lateral and vertical movement of the channel. Undammed reaches in systems affected by beaver activity are likely to become more geomorphologically complex.

Changes in geomorphological processes, and therefore channel shape and position, are also likely to occur in response to increased amounts of in-channel wood derived from beaver activity. Pieces of wood may coalesce and have a significant effect. Smaller accumulations or single large pieces may also instigate changes to both channel cross-section shape and the lateral movement of the channel by increasing channel roughness, and therefore altering patterns of erosion and deposition. Effects on habitat

The construction of beaver dams and ponds introduces many additional habitats to river reaches, resulting in a substantial increase in habitat diversity, the spatial complexity of the habitat mosaic and the overall resilience of river and riparian ecosystems to disturbances. The hydrological and geomorphological effects of beaver activity will alter the amount of lotic (running water), lentic (still water) and wetland habitat supported by a stream or river. These alterations will affect the composition of some aquatic communities, for example the diversity of lentic and lotic habitat-dwelling invertebrate species may change. The system is dynamic, with dams eventually degrading due to abandonment and/or heavy spates.

The sediment accumulating in the ponded reaches upstream of beaver dams will be sorted, with larger particles being deposited at the head and finer material in the main body. A change in the composition of bed material downstream of dams is also likely to occur as a result of sediment being retained behind dams. These changes will increase habitat diversity.

The retention of organic and mineral matter by beaver dams is likely to improve downstream water clarity and quality. The flushing of fine and sorting of coarse sediment in reaches between ponds may also occur. These effects may be beneficial for fish-spawning habitat and have provided a rationale for beaver reintroduction into degraded, incising river systems in the USA. Changes in the aquatic invertebrate community composition are also likely in response to changes in flow, sediment and food availability.

Beaver activity in streams will increase habitat diversity, notably in watercourses that have been managed. It has been demonstrated that beavers enhance habitat availability, heterogeneity and connectivity. In Scotland, as in many other parts of the world, streams and rivers have been modified by humans, for example straightened, widened and deepened, for a variety of reasons. This engineering has reduced the diversity of aquatic habitat, and therefore the species supported by it. Re-establishing the natural habitat complexity of running waters has been the focus of many restoration projects, including several that have been undertaken in Scotland in recent years (see the River Restoration Centre website). Beaver activity can hasten the restoration of habitat mosaics, and their use as initiators of recovery has been explored. One study concluded that in areas inhabited by beaver, the evolution of stream planform (i.e. the shape of the channel when viewed from above) is the result of not only physical variables but also biotic processes, for example beavers constructing dams.

A summary of the potential interactions between beavers and running waters is presented at the end of this section (see Table 4.10.1); where possible these have been attributed to a neutral, positive or negative effect.

Table 4.10.1: Summary of potential interactions between beavers and running waters



Positive effects

Negative effects



Change in riparian woodland: Opening of woodland canopy and increased patchiness

  • Development of diverse riparian understory, and therefore increase in habitat diversity and species richness
  • Increase in amount of light reaching watercourses, and therefore:
    • increase in diversity of in-stream habitat provided by aquatic plants
    • increase in geomorphological change initiated by the presence of plants (and therefore increase in habitat diversity)
    • stabilisation of banks and reduction in erosion due to binding effect of bank and riparian species
  • Reduction in shading, and therefore a potential increase in thermal stress upon some species such as fish


Change in riparian woodland: Change in relative abundance of different tree species

  • Possible reduction in type of food preferred by some aquatic invertebrates, and therefore possible indirect effects upon species such as fish
  • Possible reduction of deep-rooted species that bind bank material, and therefore possible increase in erosion


Change in riparian woodland: Change in age classes of trees

  • Possible eventual reduction in the size of wood entering watercourse, and therefore a change in the nature and scale of geomorphological change initiated
  • Possible eventual reduction in size of wood entering watercourse, and therefore change in in-stream habitat structure provided and nature and scale of geomorphological change initiated


Change in riparian woodland: Amount/diversity of fallen dead wood on woodland floor

  • Greater source of wood available to be entrained by overbank flows, and therefore possible increase in habitat diversity and likelihood of wood jams in streams and rivers

Felling and construction

Changes in amount/diversity of woody material in watercourses

  • Increased number of wood jams, resulting in:
    • attenuation of flow and lowering of downstream flood risk
    • greater geomorphological, hydraulic and habitat diversity
    • improvements in water quality as fines settle in areas of slower flow
  • Increased number of wood jams, so a possibility of localised floodplain inundation and impacts on land use


Feeding on specific terrestrial herbaceous and aquatic plant species

  • Change in nature and scale of geomorphological change initiated by the presence of vegetation
  • Change in nature and scale of geomorphological change initiated by the presence of vegetation

Dams/pond creation

Change from lotic to lentic habitat

  • Increase in habitat diversity
  • Increased flood storage, and therefore decrease in downstream flooding
  • Improvements in base flow during periods of low precipitation due to increased water storage
  • Increased fish predation opportunities

Dams/pond creation

Change in hydrological processes on riparian and downstream habitat

  • Increased habitat and species diversity
  • Increased flooding of riparian zone and beyond, so potential impacts on land use

Dams/pond creation

Changes in water quality downstream

  • Reduction in the amount of fine material deposited on bed sediment, and therefore habitat, e.g. spawning redds, maintained
  • Reduction in rate of sediment movement, and therefore the speed at which it leaves streams and rivers
  • Smothering of bed sediment upstream of dams resulting in change in habitat quality
  • Reduction in turbulence upstream of dam, so decrease in rate of water oxygenation

Dams/pond creation

Change in standing dead wood resulting from inundation of trees

Dams/pond creation

Longer term successional changes after dam abandonment, e.g. beaver meadows

  • Reconnection of streams and rivers with floodplains, and therefore lateral extension of river corridors
  • Increased habitat and species diversity
  • Improvements in natural flood management

Dams/pond creation

Impacts on movement of species

  • Dams are a possible impediment to migratory fish
  • Increased fish predation opportunities

Other construction

Creation of lodges, burrows, canals, etc.

  • Expansion in amount of aquatic habitat and attendant increase in habitat and species diversity and abundance


Indirect habitat creation/restoration initiatives as a result of beaver presence

Beavers used to promote opportunities for riparian and freshwater habitat creation/restoration

  • Beavers may be used to promote river restoration projects (as well as contributing to low-cost restoration through their own activities)

4.10.2 Distribution of suitable running freshwater habitat in the beaver policy areas Running freshwater habitat of conservation importance

To determine whether the activity of beavers on running freshwater habitat is significant in the context of this Strategic Environmental Assessment, the assessment of impacts (positive and negative) has focussed on those running freshwater sites for which beaver activity may affect directly or indirectly (as discussed above), which are considered as having conservation importance and as such are afforded European or national protection wherever they occur. Shingle Islands SSSI is the only designated running freshwater habitat that lies within beaver policy area that overlaps with potential beaver core woodland.

Table 4.10.2 shows the running freshwater habitat that lie within the potential beaver core woodland in the beaver policy areas and the site for which it is designated. See Appendix for a map of all SSSIs including Shingle Islands.

Table 4.10.2 Summary of sites that overlap with potential core beaver woodland for running freshwater habitat features of conservation importance.

Running freshwater habitat

Designated site

River shingle/sand

Shingle Islands SSSI

4.10.3 Assessment of likely effects on ecologically important running freshwater habitat in the beaver policy area

The habitat identified in Table 4.10.2. above is discussed in the context of those effects (positive or negative) that have been identified as a result of beaver activity. Where mitigation or monitoring maybe appropriate, this has been identified in the narrative. Further discussion relating to the management of beavers including mitigation and monitoring options is provided in sections 5 and 7 respectively.

For species and habitats of conservation interest in the wider countryside there will be an ongoing need to assess data derived from general surveillance and monitoring activities that are already in place, and intervene with management if and when necessary. This will be informed by a more strategic approach to management being developed in due course.

Beaver opportunities Consideration of potential positive effects on running freshwater habitats of conservation importance

Narrative is provided in section 4.10.1 above including reference to the likely positive effects of beaver activity on running water habitats in general. A summary of which is then provided in the Table 4.10.1 above. More than this, the presence of beavers in an area could provide a basis for river restoration projects (as well as contributing to low-cost restoration through their own activities).

River Shingle/Sand


There are no sites identified in Knapdale that are designated for river shingle/sand habitat.


  • Shingle Islands SSSI

SSSI Assessment

Shingle Island SSSI, comprises several islands and stretches of riverbank in the lower reaches of the Rivers Tummel and Tay. A key feature of the site is its dynamic nature. The morphology of the features changes in response to high flows and results in a continuously changing mosaic of habitat in various stages of colonisation from bare shingle and sand to alluvial alder woodland, and includes old abandoned river channels and backwaters and mixed woodland.

As beavers are unlikely to dam rivers greater than 6 m wide and the Rivers Tummel and Tay are both significantly wider than this in the vicinity of the designated site, it seems unlikely that they would have any effect. Were any of the smaller, backwater channels that flow through some of the islands to be modified by beaver activity the modifications are likely to be temporary due to the energy of the rivers in the vicinity of the designated site. Additionally any changes are likely to contribute to the dynamic nature of the habitat. Therefore, while there are natural heritage interests of national importance on this site, these are unlikely be detrimentally affected by the beaver activity.


Mitigation is unlikely to be necessary but could involve a number of beaver management options as outlined in section 5 relating to preventing beaver access to the habitat or managing impact from damming activity such as use of flow device.

As beavers have colonised this stretch of riverbank and islands of the designated site they may alter the composition of any alluvial alder woodland present. See section 4.2 Beavers and Woodland for further consideration. See section 7 for details on the approach to SCM and beavers. As Stirling University have a long history of studying the fluvial-geomorphology and habitat change of Shingle Islands, there may be scope for a more detailed study if judged necessary. Consideration of potential negative effects on running freshwater habitats of conservation importance

While table 4.10.1 above summarises a number of potential negative effects for running water habitat that could arise from beaver activity, these are not expected in the context of Shingle Islands SSSI river shingle/sand habitat.


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