Northern Prairie Wildlife Research Center
Culverts are best suited for Type IV wetlands, followed by larger Type IIIs and sheltered areas of Type Vs. Culverts should be placed within six feet of emergent vegetation in a water depth of approximately 18 inches in the spring. One structure per 10 to 20 acres is a good goal and there should never be more than one per wetland acre. Areas with nearby trees should be avoided because they provide hunting perches for raptors and crows.
(Culverts of 1.5-1.8 m in height are preferable.) A culvert can be either installed in a dry wetland or through the ice. Installation in a dry wetland is much easier and less hazardous than through the ice. To install in a dry wetland, scrape a depression in the wetland bottom with a loader bucket. Using the bucket, push down and square the culvert in the depression. While installing through the ice, use good judgment as to what the ice conditions are. If ice is thick enough to support the equipment, start by cutting a hole in the ice. Cutting a hole in the middle of the circle of ice will make it easier to lift out. Once the ice is removed, push the culvert down into the mud and level it. Try to get the culvert into solid (but not frozen) bottom substrate.
Filling the Culvert
Culverts should be filled with soil suitable for plant growth. Rock or gravel are not acceptable fill material because they do not allow moisture to reach growing plants. The soil will settle and the culverts must be revisited to replenish the settled soil. The soil can settle as much as two feet, making it impossible for ducklings and goslings to escape. Filling the culvert with water saturated fill material may decrease the settling. Plan on revisiting the site(s) at least once and probably twice to replenish the settled soil.
Culverts grow a variety of weeds from windblown or soil-stored sources. This is generally okay, but seed such as alfalfa, sweet clover, and native grasses could be spread into the soil to improve conditions. It generally takes 1 to 2 years before cover is adequate to attract nesting waterfowl. Nesting geese usually break down nearly all residual vegetation and use it as nest material. They also destroy the vertical and horizontal cover that attracts mallards. Generally geese and mallards will not occupy the same sight unless modifications are made. A partition may be placed into a larger culvert that separates geese and mallard nesting sights. The partition can be made from cedar boards (4 cm thick) to resist rot. Covering the partition with 15 cm mesh wire will allow mallards to squeeze through the mesh if necessary. A rounded opening of approximately 15 x 20 cm will provide access to the covered quadrants of the partition. Weaving 1-2 cm diameter willow sticks through the wire mesh on the side facing the open goose nesting area will ensure that the cover for the mallard nesting sight will not be incorporated into the goose nest.