There are a number of protocols and tools that have been developed to assess the potential beaver habitat For an in-depth review see Pollock et al., 2015. Depending on the nature of the restoration (pre-determined site vs. site prioritization) different assessments are possible. This section will briefly outline the considerations and questions that must be addressed in order to determine if the site is suitable for beaver translocation.
This section does not address potential threats to infrastructure or risks associated with translocating beaver. There are locations in which beaver should not be translocated due to the potential for flooding or damaging infrastructure. Those risks, and how to evaluate and mitigate them are addressed in Managing Risk
Beavers build dams in order to meet create habitat that increases their survival by providing underwater access to their lodge, foraging opportunities, and a winter food cache while reducing the risk of predation. Not all beavers build dams. Along larger rivers (e.g. the Colorado River) beavers may build lodges in banks which still provide underwater lodge access. Because the dam building activities of beaver are what influence the processes that interest many restoration projects, this discussion is limited to dam-building beaver.
In order for beaver to persist in the landscape they require forage, building materials, and a flow regime that will allow their dams to persist.
Beaver are ‘choosy’ generalists, meaning they can survive on a variety of different species, but preferentially choose certain species. Aspen and willow in particular represent highly desirable forage for beaver. Beavers’ diets also change seasonally, their winter diets are dominated by woody vegetation that has been harvested and cached within their ponds, while summer diets may be dominated by herbaceous vegetation. In addition to its importance as forage and building material woody vegetation is essential to meet the needs of beaver physiology. Beaver incisors grow continuously and woody vegetation is necessary in order for beaver to maintain their incisors.
Beaver dams are built from a combination of woody vegetation and sediment. While willow, aspen and other woody riparian vegetation are often thought to be required for dam building observations in arid and/or degraded riparian areas where those materials occur in lower amounts other woody upland species, such as sagebrush have been observed in beaver dams as well. During the site assessment both the quantity and quality of woody vegetation needs to be assessed. For example, willow that are limited in size due to conditions of grazing or degraded hydrology are lower quality building materials than larger willows, which has implications for dam persistence.
Beavers require a perennial source of water to survive. While there have been cases where beaver have caused a change from intermittent to perennial streams, when considering sites for beaver translocation there must be a perennial water source. Equally important, annual flow must not be so high as to cause dams to breach, or prevent their initial construction. In other words, beaver must be able to build dams at baseflow (perennial, but not too high) and dams must be able to survive annual peak flows.
It is further important to note that dam persistence depends on the local geomorphic setting, and is not solely a function of the flow regime. See Beaver Restoration Assessment Tool
Assessing a restoration site’s suitability to support beaver can be performed in the field. In order to make such an assessment it is necessary to some reference condition to inform whether or not there is sufficient forage and building material to support beaver. Assessing flow regime relies on channel morphology characteristics that can act as surrogates for discharge. Additional information regarding the historic presence or absence of beaver and the reasons for their current absence are also helpful in assessing whether a site is suitable for beaver reintroduction.
Where the riparian resources and flow regime are capable of supporting beaver but the geomorphic setting is preventing persistent beaver dams (as in incised channels) using beaver dam analogs provides a way to promote persistent dams by 1) direct construction of stable structures and 2) altering hydraulic in order to create the geomorphic setting that allow beaver dams to persist.
Evaluating hundred or thousands of stream kilometers in order to identify and prioritize restoration sites requires using remote sensing data before going into the field. While the tools required for such an analysis are different the overall approach remains the same. Managers wanting to evaluate large spatial extents require information about 1) vegetation 2) flow regime and 3) geomorphic setting.
During a field assessment managers can reasonably ascertain whether or not physical factors such as slope may prohibit dam building (i.e., a waterfall or slopes > 0.10 m/m). Therefore, in order to assess the physical capacity of an area to support beaver dams data sets that address 1) vegetation/land cover 2) discharge and 3) topography are required.
The Beaver Restoration Assessment Tool (BRAT) is a decision support and planning tool that allows restoration managers to assess the potential for beaver as a restoration agent over large spatial extents. The BRAT model predicts the capacity of rivers to support beaver dams based on vegetation, topography, and streamflow variables. In addition to a current estimate of dam capacity, it predicts the historic capacity and by including additonal data (e.g., transporation infrastructure)it predicts the potential conflict due to dam building activities. Finally, by combining the capacity output and the conflict potential output the model delineates “beaver management zones” that identify the areas most suitable for restoration with beaver. Currently, the BRAT model has been run for the state of Utah. Results from the model and more information can be found at BRAT website and/or paper