Beaver Defeaters

Damming is a common problem with culverts in beaver country. To a beaver, a culvert under a roadway looks like a hole in an otherwise perfect dam. So they plug them, often backing up the stream until the water flows over the roadway.

Usual methods of control include constant culvert maintenance, beaver trapping, or the insertion of a pipe through the culvert and the beaver plug dam on the upstream end. These pipes installed in various ways are known as beaver deceivers.

The reason beaver deceivers work is that the beaver dam building is prompted by the sound of running water. If the beaver deceiver pipe is angled through the culvert and poked through the plug dam so that the upstream end is always under the water's surface, the sound of running water will only come from the downstream end of the pipe. Rarely are beavers successful at stopping water from flowing out of a pipe, as opposed to stopping water from flowing into a pipe.

These pipe-within-a-pipe beaver deceivers are almost invariably fish barriers. Because of their angle of installation in relation to the streambed, they are often perched. Also because they are smaller in diameter than the culverts they are threaded through, they usually become velocity barriers as well.

We have installed and designed a number of "beaver defeater" exclusion fences around problem culverts in our watershed. These structures are constructed of 16 x 4 ½' galvanized cattle fence panels. Their purpose is to prevent the beavers from transporting material to the mouth of the enclosed culverts.

The mesh size of the cattle panels is 8 x 6 "; large enough to pass fish but small enough to foil stick toting beavers. The beaver defeater fences are triangular in shape with the narrow end enclosing the culvert end, extending a minimum of 16 feet upstream of the culvert, and flaring out to a minimum of 16 feet in width. The downstream end of the culverts are also grated off with fence panel and backfilled to maintain the desired beaver pond elevation.

Backfilling at the downstream end of the culvert is essential. Backwatering the culvert prevents the flow noise that is the beavers' dam-building impetus. To date the structures we have installed have been successful; no beavers have had the inclination to dam the entire wing fence structure.

Material costs for these units run about $200/each. Installation usually takes 16-20 man/hours. Private landowners that have seen the units are switching over to the beaver defeaters as they take less maintenance than other control methods.



Copyright 2001 Stillaguamish Tribe of Indians