Site Selection Criteria
Beavers may be released into the wild, or they may be released into very large outdoor enclosures. Here we list some of the key criteria that should be considered for selecting a trial reintroduction to the wild, based on Gurnell et al. (2008).
- Minimum length of river – a 2 km length of river with suitable habitat should be sufficient to support a colony of beavers, where habitat is assessed within 20 m of the water’s edge.
- Channel / Floodplain Downstream Gradient- this is the most important physical factor influencing beaver occupation, partly because it is a key control on whether dam construction is necessary to create appropriate habitat. Slope also dictates river energy (and velocity) and so is closely associated with bank materials and flood plain construction/extent. Research on Canadian beavers clearly demonstrates that slopes > 0.15 are unfavourable for beaver and that slopes <0.06 (preferably <0.03) are favourable.
- Bank profile and materials – since European beavers tend to use bank lodges as den sites, the height of banks and the materials from which they are built are important habitat characteristics. Habitats that contain areas of relatively high banks (>1.5 m above baseflow water level) built of relatively fine materials (earth / loam / peat) are ideal, although shallower (>0.5 m) coarser (fine gravel) banks could support beaver. Because of the preference for fine bank materials, rock type is likely to be limiting and certainly hard rock areas are likely to be unattractive to beaver unless there are significant fine alluvial deposits in valley bottoms. Bank slope is sometimes specified within a list of beaver habitat requirements, but if a river meets the bank height and calibre requirements, it will support a variety of bank slopes and so slope is unlikely to be limiting.
- Water velocity and depth – water depth and velocity are both important but are also interrelated aspects of beaver habitat. Their modification underpins dam building activity. Water depth adjacent to the beaver lodge needs to be sufficient for burrow/lodge entrance to be reliably maintained below water level. A general water depth >1 m should be sufficient, but >2 m is preferable. If sufficiently deep water is present then velocity is likely to be low, at least during periods of baseflow.
- River width – although river width is often reported as a beaver habitat requirement, it is unlikely to be critical. Rivers with low slopes, deep water and high, fine calibre banks are usually wider than those with steeper slopes, shallower water and low, coarse calibre banks. Thus the correlation between river width and beaver occupation does not really indicate dependency.
- Floodplain characteristics – floodplain width gives an indication of the degree to which beaver construction of dams and canals could extend their habitat laterally. It could be an informative but not crucial habitat variable to consider. However, the presence of lateral water bodies (side channels including artificial ditches, backwaters, cutoffs and other flood plain ponds) greatly increases the suitability of a river reach for beaver occupation.
- Vegetation - beavers are vegetarian and depend upon vegetation for food and construction materials. Good herbaceous vegetation provides summer food but tree bark is important in winter. Therefore good beaver habitat supports a mix of herbs and favoured tree species (Salicaceae and Betulaceae) within 20 m of the water’s edge. Whilst the ideal is approximately 50% closed tree canopy and the ideal tree/shrub dimensions are diameter at breast height (dbh) < 0.15 m, beaver manipulate the vegetation, felling large trees which resprout with smaller shoots and open up the canopy. Thus the crucial element of river margin vegetation is the presence of a good cover of favoured tree species.