Northern Prairie Wildlife Research Center
|Figure 2. Islands built in an oval shape with a rounded outline create a more natural appearance.|
To minimize costs, islands should be constructed where the water depth seldom exceeds 3 feet. Water-level information might be obtained from local residents or by locating previous high-water marks. Ideally, construction should take place when the wetland is dry or in winter when there is sufficient ice to support heavy equipment. All ice and vegetation must be cleared from the island base during site preparation.
In open water habitat, Ducks Unlimited recommends that the island base be as high as the average water level in the wetland and constructed with 10:1 side slopes (Figure 2). The top of each island should rise 4 feet above the base and have 4:1 side slopes. A 10-foot-wide, flat berm is usually constructed between the bottom of the island slope and the top edge of the base slope to absorb wave energy and to slow island erosion. Where islands are built in wetlands with moderate wave action, a single 6:1 or 8:1 side slope without a berm is acceptable.
The island should be constructed with soil or fill from the wetland bottom immediately adjacent to the construction site or from an upland borrow area. Because mink are attracted to permanent, steep-sided ponds, excavated areas in wetlands should be no more than 1 or 2 feet deeper than pond bottom. Most suitably, fill should be taken from an area immediately around the island, leaving an excavation with 20:1 or gentler side-slopes.
|All ice and vegetation must be cleared from the island base during site preparation. (photo by Ducks Unlimited)|
The mid-wetland sites recommended for islands subjects them to severe wave action. Preferred soils for island construction should contain about 30% clay mixed with silt and sand and preferably some aggregate. Fill material should not include chunks of ice. Prior to moving fill material from the borrow site, topsoil should be removed and stockpiled for later use.
During construction fill material should be deposited in a continuous, layering fashion. Each layer of material must be thoroughly compacted before another layer is put into place. Upon completion of construction, the previously stockpiled topsoil should be spread 4 to 6 inches deep across the surface of the island. When construction is completed, haul-roads should be removed and the entire area reshaped to restore the original contours.
|During island construction, fill material should be deposited in a continuous, layering fashion. Each layer of material must be thoroughly compacted before another is put into place. (photo by Ducks Unlimited)|