Beaver Institute™

A Keystone Culvert Fence

Beavers like to build their dams where the least amount of work will result in the largest impediment of water. To a beaver a culvert in a roadbed probably looks like a hole in a dam. So road culverts are very attractive damming sites for beavers. With a little work to block the culvert the entire road bed becomes a dam. This can result in serious road flooding and damage, safety issues, and expensive repairs.

Fortunately though, nearly every road culvert can be protected from beaver damming. Some of the flow devices used to protect road culverts include: large fence systems, diversion dams, or fence and pipe flow devices. Which method is best suited to protect a culvert is dependent upon site specific characteristics such as roadbed height and composition, tolerable water depth upstream of the road, stream flow, potential for abutter flooding, and fish and wildlife passage.

Usually the most cost-effective, long-term, humane and environmentally friendly option to resolve beaver damming problems at road culverts is to install a well-designed flow device. Effective flow device designs include: diversion fences, large exclusion fences, and Fence and Pipe systems.

Diversion Fences and Dams

A Diversion Fence or Dam is the easiest and least expensive way to protect a road culvert from damming. It is very similar in construction to a Beaver Dam Analog, but much smaller. It is best utilized where there is a high road bed and no low lying human development so a beaver pond is tolerable provided the culvert remains open.

There are times when a stream channel at a road culvert is too narrow for a trapezoidal Keystone Fence™ but a beaver dam outside of the culvert would not pose a threat to the road. In situations such as these, building an upstream diversion fence or dam may be the best way to protect the road.

Beaver Institute

Dammed Diversion Fence Protecting a Road Culvert in Andover, MA

The diversion fence or dam is typically constructed across the streambed about 10 – 12 feet upstream of the culvert, and needs only be built a foot or so above the stream level. Use small mesh fencing and place leaves, grasses and branches against it so it starts backing up water.

The idea is to create a small impoundment of water and some cascading water noise. This pool of water and the water noise will stimulate the beavers to dam on top of the diversion fence rather than in the culvert. It takes advantage of the beaver’s preference to build in the easiest spot.

If large stones are readily available on-site use them to build the diversion dam. Once again you want to create a small pool of water immediately upstream of the culvert. The idea is to make damming on the rock wall easier than blocking the culvert. More often than not the beavers will start building their own dam on top of your construction. Positioning your diversion dam 10 – 12 feet or so upstream of the culvert ensures the branches on the backside of the beaver dam will not be partially blocking the culvert.

In the future, if the size of the beaver dam needs to limited, a Fence and Pipe flow device can be installed on the culvert to limit the size of the beaver dam.

Diversion Dam Info Sheet

Culvert Protection Fences

Blocked Road Culverts – The most common beaver flooding problem
Fortunately, properly designed and constructed culvert protective devices have proven to be extremely effective in eliminating culvert damming by beavers. The trapezoidal shaped fence was invented by Skip Lisle with the Penobscot Indian Nation in Maine which he trademarked “The Beaver Deceiver”. His Beaver Deceiver and similar culvert fence designs flow are extremely effective keeping culverts from being dammed by beavers. They eliminate beaver culvert damming in several important ways.

First, the fence creates a much longer distance than the width of the culvert that the beavers must dam. Effective culvert fences are typically over 40 feet long making it much more work and less desirable to dam at the culvert, so the beavers will choose to dam elsewhere, keeping the culvert open and the road safe from flooding.

Second, if beavers do try to dam the culvert inlet, the Culvert Fence will force them to dam further and further from the culvert. This misdirection also discourages damming on the fence.

Third, fence should be built in a trapezoidal shape provided water will surround the entire fence perimeter. The trapezoidal shape discourages beaver damming because if the beavers dam on the fence they will get further away from the culvert inlet, effectively widening the area that the stream is flowing into. This widening of the water inlet reduces the water movement through the sides of the fence. Since the sound and feel of moving water are strong damming stimuli for beavers, their desire to continue damming is reduced by the decreasing water movement stimulus.

For these three reasons, culverts can be effectively protected from beaver damming with a properly designed Culvert Fence.

Six inch by six inch, 6 gauge concrete mesh fence is optimal. The openings are large enough to minimize floated debris buildup on the fence, yet small enough to prevent beavers from carrying damming materials inside. Fence flooring is often installed to prevent tunneling under the fence. Quality construction with the finest materials is imperative if the system is to withstand the test of time, the elements, and the beavers.

Maintenance on a well-designed culvert protective fence is minimal and typically consists of three to four times a year removing sticks and leaves that floated against the fence. This usually takes less than 15 minutes per cleaning and can be done by anyone willing to get in the water.

More Culvert Protection Info Sheets

Fence and Pipe Device Info Sheet

The Keystone Fence Info Sheet


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