Northern Prairie Wildlife Research Center
Islands are most easily built when the pond is being constructed or restored (Figure 1). The above water portion can be any size from .01 acre (20-foot diameter) to 2 acres, but 0.1-acre to 1-acre islands appear best. Larger islands are not recommended because they can harbor nest predators year-round. Islands should be placed at least 50 feet from shore and be surrounded by water that is a minimum of 2 feet deep to make predator access difficult. The top of the island should be 3 feet higher than the anticipated high water level or one to two feet above the emergency spillway level. Side slopes should be no steeper than 5:1 in exposed locations, but could be 3:1 in sheltered spots or small ponds. Riprapping the edge of the island may be necessary in open water to prevent erosion. Islands should initially be seeded with a temporary cover crop of rye or oats until dense permanent vegetation such as canary grass, switchgrass, buck brush, gooseberry, raspberry, dwarf spirea or dwarf willow can be established. Geese and ducks often nest closer together where brush or tall grass creates a visual barrier that reduces territorial fighting.
The number of islands constructed depends upon the size of the pond and the size of the islands. We recommend that the ratio of island acres to surface water acres be about 1:50, with no more than 1 small island per acre of water. Islands should be at least 150 feet apart.
If you intend to build an island in an existing wetland, you may need to obtain a 404 permit from the Army Corps of Engineers. Application forms are available from your local wildlife biologist.