Heidi Perryman
Discerning beaver watchers have noticed something new in the water over the past few days. Another little dam. Kindly and protective souls have wondered if this should be kept secret from the city, immediately removed without their involvement, or maybe covered up with water lillies so that it doesn’t provoke a general response of panic. I photographed the new structure and sent the pictures around to our beavy of beaver experts, including Skip Lisle. The general response was that this was likely a “secondary dam” not a relocation of the first one. Secondary dams are usually down stream from the “main dam”, and form what Skip calls a kind of “terrace effect” for the pond.
They are are lower in height and primarly help by backing up water against the main dam for extra support. Here’s what authors Steve Boyle and Stephanie Owens say in their paper prepared for the CO forest service: North American Beaver: A Technical Conservation Assessment.
Beavers usually build one “home” dam to create a pond for the main lodge. Secondary dams may also be built depending on food availability, soil type, and terrain (Rutherford 1964). Sometimes these dams are built at different levels, resulting in a “lock” system to control water at each level. Secondary dams allow beaver to transport food and materials to the home pond over greater distances while minimizing travel over dry land (Rutherford 1964). Beavers often construct extensive canal systems in shallow ponds or wetlands to aid in the transport of cut wood (Novak 1987).
The beavers continue to use their lodge, and work on the main dam. Indications are very strong that this will be maintained and that the beavers effort in this secondary structure will be minimal. Remember that the influence of tide gets much stronger the farther downstream they go, so the beavers success will be mitigated by very strong forces. You can tell from the patterns of the rushes that they are using this area to feed and forage.
Secondary dams are a sign of beavers doing what they are supposed to, settling in and adapting to their life.
They are not a cause for alarm, and there is little point to their removal since the beavers will likely rebuild. Since their ultimate water height is controlled by the main dam they do not present a flooding danger and will probably never need a secondary flow device.
And, as one clever beaver supporter mentioned to me, they have again chosen an area with a bridge for optimal viewing. Pretty smart beavers.
