Mitigation of the impacts of the beaver Castor fiber

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Ann-Marie MacMaster, Rivers and Fisheries Trusts for Scotland (RAFTS)

Beaver typically build dams in shallow burns or streams (rather than large, deep rivers) in order to raise the water level so that they can swim, feed, cache food and enter the lodge in relative safety.  The impressive engineering skills of the beaver together with materials used, such as branches, soil and plants, collectively ensure that the dam is a substantial semi-permanent structure.  Once the dam is built the water level rises and it is possible that the immediate surrounding habitat may become flooded.

Photo 1: Pipe inserted through dam in order to reduce the water level.  Note mesh cage at one end to prevent debris from blocking the pipe © Ann-Marie MacMaster

Photo 2: Mesh netting can be used to protect trees from beaver damage © Aylwin Pillai

Anecdotal evidence suggests that a number of species benefit from the activities of the beaver including plants, dragonflies, birds and amphibians.  In addition, beaver areas slow down water run-off during flooding, decrease peaks of flooding downstream and beaver ponds trap nutrients and retain silt.

Not everyone will benefit from the beaver however.  Economically valuable land such as arable land and forestry are at risk from damage.  In such cases there are a number of non-lethal measures that can be used to deter beaver from living and foraging in a given area.


  1. A pipe(s) can be incorporated through the dam allowing water to drain away faster and the water level to drop. A cage is constructed around the mouth of the pipe so that any debris in the water does not block the pipe entrance (see photo 1).
  2. Trees can be felled by beaver for both food and dam construction materials. In cases where valuable trees are at risk from beaver damage mesh netting or sheets of galvanised metal can be used to protect the tree trunks before beaver begin felling trees (see photo 2).  An alternative to this is to paint the tree trunks with ‘anti-beaver paint’ (a mixture of sand and glue which deter beaver from damaging the tree).
  3. Electric fencing can be used to prevent beaver access (e.g. fishing ponds or crop fields) however this can be expensive and time consuming to maintain as the vegetation must be kept in check in order for the electric fence to remain in good working order.
  4. If flood prevention banks are at risk of damage by beaver burrowing behaviour, galvanised steel netting can be used to prevent the beavers from burrowing. The bank can also be in-filled with concrete.
  5. Scaring devices may also be used to deter beaver from a given area. These could be visual (e.g. flashing lights) or audible (e.g. loud noise).
  6. Non natal beaver dams, e.g. dams which are built to enable the beaver better access to food but which are not essential to their survival, can be removed.
  7. In some cases lethal measures must be taken. Where human health and safety are at risk, e.g. beaver burrowing beneath a railway embankment or road, then it is possible to remove the dam (including natal dams) and lodge and the beaver will also be removed by either trapping or shooting on sight (see photo 4).


Photo 4: Where human and health and safety are at risk it is necessary to trap and Cull  beavers © Ann-Marie MacMaster



Beavers and Flood Prevention.                                                        Jim Perrett

The impact of Beavers on flood banks is a major problem in Bavaria. We visited two sites where such lodges existed. They burrow into and then upward within the banks.

This action obviously has potentially disastrous effects on the integrity of the flood bank.

Simple removal of the lodge and backfilling the damaged bank is not worthwhile as the beaver would only find another point nearby and start again. Such action is also prohibited by Article 12 of the Habitats Directive which prohibits the deliberate disturbance of protected species and the damage or deterioration to their ‘breeding and resting sites’ Action to remove the lodge or the animal requires a derogation (or exception) under the Habitats Directive. This is implemented in Germany through the Federal nature conservation law and, for Bavaria, through Bavarian state law, the AAV.

The Bavarian answer is to try to implement measures to protect flood banks from beaver activity.

Where an existing burrow has been identified within the bank, reinforcing the dam with sheet piling on the inside face of the flood bank forms a barrier to stop further incursion ( option 1)

When it is found that a bank is in general disrepair then when the bank is rebuilt a membrane of clay with mesh is inserted.(option 2).

If there is no beaver activity and the bank is deemed sound then preventative action may be carried out by revetment of the banks using large stones.

Where beavers have already burrowed into flood protection banks remedial measures can only be undertaken as an exception to the legal protection afforded to the beaver (as above) and this must be in compliance with the tests for a derogation under the Habitats Directive (serious damage, no alternative course of action and not detrimental to the favourable conservation status of the beaver).

Flood banks in Scotland generally fall into two categories. The Government/Local authority Flood Prevention Schemes e.g. Perth, Bridge of Earn, and Weem, and the flood banks like those on the Isla, Tay and Dean water (which are primarily for agricultural land) whose upkeep lies with the riparian owner.

Generally we find that in new design Flood Prevention Schemes that the banks are well back from the river or stone faced so beaver incursions should be rare though future design work should take possible beaver incursion into consideration.

The older farm schemes had a tendency to maximise the available land use and were therefore prone to be on or immediately adjacent to the river bank. Work on these flood banks is usually in response to there failure in a flood and the repair work falls to the riparian owner. An inclusion of a beaver barrier under these circumstances would be pointless. Weakening by beaver incursion will increase the failure rate. Should a major length of bank require replacement then the inclusion of beaver membrane would add extra cost but not a significant percentage of the total.

Both the possible increase in bank failure and the required installation of bank reinforcement where there is known beaver activity will have costs to the riparian owner which would have to be addressed through some form of Grant Scheme.



Beaver dams and Flood attenuation.

The subject of attenuation was discussed briefly with a representative of the Ingolstadt Water Management Authority. He had no particular view on the subject.

My own view would be that in a single stream scenario the dams could be large enough to have an effect of lowering the peak flow from a flood, by how much would depend on the dam area. However whether a number of dams on numerous side streams of a major river would have a similar effect, the juries out.

It would depend where the dams were sited relevant to the main stem. Attenuation could work both ways.

If all the dams were in the headwaters, and none in the lower reaches, then they would aid in the lowering of the flood peak. This can be seen quite graphically if you compare the flood flows from the rivers Lyon and Tay at there confluence where Loch Tay attenuates the River Dochart and River Lochy flood flows.

If all the dams were in the lower areas of the river basin they could increase the flood peak by attenuating flows that would normally have dispensed prior to the main stem flood peak.


Beavers and Migratory Fish.

I have been an active member of the River Earn Improvement association for over 25 years and a Hydrologist for over 40 years most of which was covering the Rivers Tay, Earn, Eden and Esks, and latterly also the Dee Don and Deveron. I would wish to believe that my understanding of migratory fish movement and of river Hydrology puts me in good stead to make the following deductions on the effects of Beaver Dams on fish migration.

The group visited beaver dams in Bavaria on tributaries of the Danube. A number of the dams were on drainage ditches on a large flood plain so weren’t really a concern regarding migratory fish. However on streams that I could make a comparison with to similar tributaries of the River Earn the dams would be generally impassable to migratory fish. The exception was on the Esau River where a series of dams had well defined spills. We were informed that this particular structure had been in existence for around 15 years.

With structures up to 1.5metres high in the existing stream channel and stretching over the flood plain, up to 100metres in one case, it’s doubtful if they would be drowned out sufficiently to allow upstream passage of Salmon or Trout. My experience has shown that where a restriction does exist Salmon would simply drop back downstream rather than try to ascend the structure. A case study is my local burn where a 1 metre high pipe bridge was built some years ago. No Salmon fry or parr were found above this structure during a recent juvenile survey. Salmon and Seatrout were regularly present prior to the works being carried out.

There may be exceptions where upstream migration of a small numbers of fish is successful. We then have the problem of the downstream smolt migration. If river levels remain low then the smolts will gather in the dam pool. This will make them easy targets for predators. Should a smolt be fortunate enough to actually crest the dam it more likely to find itself in long grass and reeds with no obvious channel to the stream. Therefore one can assume a very high mortality rate.

With sea trout numbers already a cause for concern. The smaller tributaries being there main spawning areas any dams will have a serious impact on the population.

The obvious answer to the dams is there removal. The difficulty here is by doing so we breach Article 12(1)(b) of the Habitats Directive prohibits the deliberate capture or killing species and the ‘deliberate’, as opposed to accidental, disturbance of Annex IV species

For anglers or Fishery Boards to remove dams an exemption to the Directive would have to be sought by the Scottish Executive. This could be site specific where local fisheries boards classify streams using existing data of population distribution. The Class (1) streams having total protection against beaver incursion.

Tree felling and bark ringing was prevalent in established territories. The impact on the river channels would be minimal but could be more of an inconvenience to fisheries.

The effect of dams on water quality has been quoted as possibly beneficial. I don’t profess to be knowledgeable on this front. They will certainly act as a settlement pond allowing deposition of some silt and improve downstream turbidity. My only concern is by doing so they may be detrimental to invertebrates in the immediate area downstream of the dam. I believe that there is some investigative work being carried out in Norway.

There is a paper circulating, from the University of Southampton which purports to increased habitat for fish in beaver dams supported by a number of Fisheries managers. It should be noted that this will not include Salmon Fry and Parr as they do not frequent this type of Habitat and is more likely to refer to course fish and brown trout.


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