JBL Environmental Services Ltd.

How to Coexist with Beavers?

The American beaver (Castor canadensis) is a keystone species that has a disproportionately high influence on its environment. Beavers modify stream and riparian landscapes by building dams and harvesting trees. These activities create wetland complexes and openings in riparian forests that increase the diversity of plant, bird, mammal, insect, and fish communities (Dittbrenner et al. 2018, Pollock et al. 2017).

In urban (and rural) areas, however, beaver activity can create risks to public safety on public and private property, especially during rain events. Beaver dams on narrow, channelized urban streams can quickly cause water to rise beyond the streambank, which leads to flooding and property damage. Flooding weakens tree roots, and in addition, beavers remove riparian trees and vegetation for food and building material. Salmon are culturally and economically important, and salmon access to spawning and rearing grounds is a concern in urban areas.

Beaver management in the U.S.A. and Canada is moving away from reactive techniques (i.e., dam removal, live trapping and translocation/relocation, and lethal removal of beavers) to proactive techniques (i.e., coexistence techniques and infrastructure upgrades). Selected coexistence techniques are summarized below.

Examples of Coexistence Techniques
Flow-leveling Devices: to limit unwanted flooding and control upstream pond stage height

  • Mechanical flow-leveling devices, commonly referred to as flow levelers or pond levelers (Figure 1 and Figure 2), can be installed through the active dam area.
  • Projects on the Pacific Coast have successfully used single or double pipes of 12-18 inches (30-46 cm) in diameter and up to 40 ft (12 m) in length to control flooding from beaver activity.
  • To be successful, flow-leveler installations need to leave a sufficient water depth to cover the opening of the beaver den (or lodge), providing a minimal channel depth of 1.0 m for the beavers. In cases where flow levelers failed, the reason was that beavers constructed additional dam(s) because water levels were not sufficient (Pierce 2016, Pollock et al. 2017).
  • Flow devices will require post-installation maintenance to remain effective and not all site characteristics (e.g. topography, development) are conducive to flow-device installation (Taylor and Singleton 2014, Hood et al. 2018).
  • Stream flows must be calculated before installation and compared to the conveyance capacity of available pipes (Vanderhoof 2017). If the pipe is too small, rain events will overwhelm the flow-leveler and still cause flooding. If the pipe is too big, too much water will run through the pipe and the dam will not overtop during rain events, potentially restricting downstream movement of fish.

Flexible Pond Leveler (fish-passable) – Figure 1:

  • This leveler uses a flexible pipe and cylindrical fence; the fence blocks the pipe entrance in a way that the beaver cannot detect the water flowing into the pipe and won’t plug it.
  • To be successful, flexible levelers must maintain a water level low enough to resolve the threat from flooding, but high enough to enable the beaver to access its underwater den. This discourages the beaver from building additional dams downstream.
  • This design has been shown to enable passage of salmonids (shown by the Public Works Department of Snohomish County who did not encounter fish-passage impediment (for chum salmon) after installing 50 flexible levelers (Pollock et al. 2017).
  • Improper placement of the downstream end of the pipe, however, has been found to impede the movement of fish across this leveler. The pipe needs to be placed in a pool on the bed of the channel (Pollock et al. 2017).

Figure 1. Flexible Pond Leveler installed through the beaver dam (Image and Notes: WDFW 2011)

Snohomish Pond Leveler (fish-passable) – Figure 2 and Figure 3:

  • The Snohomish leveler uses the flexible design (as described above) but includes a fishway below the dam. An optional one-way door exit in a mesh cylinder, attached to the exit pipe, can be used to guide salmon from the pipe to the upstream side.
  • The fishway assists adult salmon passage because (1) the pools provide resting places (eddies) before the salmon continue upstream into the 40 ft (12 m) flow leveler pipe, (2) each pool dissipates water energy, creates eddies, and decreases the water velocity in the pipe by 50%, and (3) pool 2 backfills the entire pipe with water (Beaver Solutions LLC 2010). The box fishway can be installed during the spawning run and removed when not required to reduce damage and maintenance costs.
  • The Snohomish leveler is intended to prevent flooding while facilitating upstream adult salmon migration through fish-friendly adaptations at both ends of the leveler. The installers who tested the Snohomish Leveler have named this device the most cost-effective, long-term, and fish friendly solution to flooding caused by beaver activity (Beaver Solutions LLC 2010). Unfortunately, it has not been widely tested in a variety of flow conditions and stream gradients and is currently in limited use (Vanderhoof 2017).

Figure 2. Snohomish Pond Leveler includes a fishway (Image: Beaver Solutions LLC 2010)

Figure 3. Two-slot fishway diagram passable to adult salmon (Image: Beaver Solutions LLC 2010)

Culvert Fencing: to prevent beavers from building dams at culvert inlets or inside culverts.

  • Sturdy trapezoidal mesh fences (such as the “Beaver Deceiver” originally invented by Skip Lisle in 1996) that are installed on the upstream side of a culvert.
  • The trapezoidal fence (Figure 4) forces a beaver that starts damming the culvert to move further and further away, thereby maintaining unrestricted flows into the culvert. The wider fence at the inlet also reduces the sound of water, thus reducing the beaver’s instinctive dam-building behaviour to try to block the flow.
  • Design considerations (Vanderhoof 2017): (1) fences must be dug far enough into the substrate to prevent tunneling by beavers, (2) the fences work better in deeper water because beavers need to pile more debris to restrict water flow, (3) a fenced-off area of 10-20 ft (3-6 m) is effective to block access to the culvert from all sides, including from shore, (4) mesh openings must be large enough to enable fish passage of all species and life stages, (5) regular maintenance will be required, and (6) downstream fencing might be required to prevent beaver access to the culvert.
  • Fencing in urban watersheds is uncommon due to the high maintenance requirements and the risk posed by debris accumulation and subsequent flooding.

Figure 4. Trapezoidal culvert fencing installed on the upstream side of the culvert (Image: Vanderhoof 2017)

Tree Wrapping: to protect vegetation from gnawing by beavers

  • Two ways to protect vegetation from beavers include enclosures for multiple trees such as exclusion/field fencing and wrapping of single trees with wire fencing.
  • Exclusion fencing usually consists of metal fence posts supporting galvanized wire mesh (4 x 9 cm) to protect larger vegetated areas. This type of fencing is less useful in large, densely planted areas because of the required material and labour. Field fencing with rolls of (1-inch-diameter) wire mesh is a less expensive (but also less sturdy) alternative for large areas. Exclusion fencing requires a height of 1.2 m and a 46-cm skirt to prevent beavers from burrowing underneath.
  • Wrapping of individual trees is done with sturdy (14-gauge) galvanized cylindrical cages (i.e., mesh with 4 x 9 cm openings) that are fastened together while leaving a space (~15 cm) between the fence and the tree trunk. Cages should be anchored to the ground at the base of the tree.
  • Tree wrapping for coexistence with beavers needs to balance the need for tree protection with the need for an adequate beaver food-supply.

Closure
The key concerns related to urban beavers are flooding (caused by dam building and/or culvert-plugging), the harvesting of trees within the riparian zone (caused by felling, and drowning of trees from long-term flooding), and reduced fish passage, particularly of salmonids.

The installation of flow-leveling devices can mitigate flooding impacts caused by beavers. Flow-device installation, however, requires analysis of the creek conditions and threshold water-surface elevations, and a commitment to monitoring the short- and long-term benefits and challenges over time. Beaver coexistence techniques in urban watersheds with high salmon values are relatively new. More research and study are needed to determine the effectiveness of beaver coexistence techniques in urban systems.

JBL Environmental Services Ltd. is interested in partnerships with municipalities in the Lower Mainland to develop a pilot project that studies the effectiveness of flow-level devices in enabling fish-passage (particularly of Chum salmon) at different stream/pipe gradients. We would be pleased to provide you with a proposal that outlines the scope and workplan for the pilot project.

The content of this brochure was researched and compiled by Andrea Paetow (M.Sc., R.P.Bio.) and John Black (B.Sc., R.P.Bio.), JBL Environemental Services Ltd., http://www.jbl-env.com, Burnaby, BC. Please contact us for more information at jblack@telus.net or apaetow@telus.net.

Literature Cited
Beaver Solutions LLC. 2010. Snohomish Pond Leveler. Team members: M. Callahan, B. Dittbrenner, J. Jacobson, D. Kolbe, T. Parker, M. Rustay, and Y. Chait. Online: https://www.beaversolutions.com/.
Dittbrenner B. J., M. M. Pollock, J. W. Schilling, J. D. Olden, J. J. Lawler, and C. E. Torgersen. 2018. Modeling intrinsic potential for beaver (Castor canadensis) habitat to inform restoration and climate change adaptation. PLoS ONE 13(2): e0192538.
Hood, G. A., V. Manaloor, and B. Dzioba. 2018. Mitigating infrastructure loss from beaver flooding: a cost-benefit analysis. Human Dimensions of Wildlife 28(2):146-159.
Pierce, J. R. 2016. Understanding and managing beavers in Vancouver. Report for the Vancouver Park Board and City of Vancouver.
Pollock, M. M., G M. Lewallen, K. Woodruff, C. E. Jordan, and J. M. Castro (Editors). 2017. The beaver restoration guidebook: working with beaver to restore streams, wetlands, and floodplains. Version 2.0. United States Fish and Wildlife Service, Portland, Oregon. 219 pp. Online: https://www.fws.gov/oregonfwo/promo.cfm?id=177175812.
Taylor, J. D. and R. D. Singleton. 2014. The evolution of flow devices used to reduce flooding by beavers: a review. Wildlife Society Bulletin 38(1):127-133.
Vanderhoof, J. 2017. Beaver management technical paper #1: Beaver management tools literature review and guidance. King County, Water and Land Resources Division. Department of Natural Resources and Parks, Seattle, Washington.
Washington Department of Fish and Wildlife (WDFW). 2011. Living with wildlife: beavers. Adapted from “Living with wildlife in the Pacific Northwest” by R. Link (2004), Online (WDFW): https://wdfw.wa.gov/living/beavers.html#preventingco