Union Slough National Wildlife Refuge was established in 1937 to provide a refuge and breeding ground for migratory birds and other wildlife. The actual slough is all that remains of a pre-glacial riverbed, and its name is derived from the connection or "union" of two watersheds: the Blue Earth River of Minnesota and the East Fork of the Des Moines River. The terrain is nearly flat, allowing the flow of the water to be determined by the direction of the wind at times. The refuge is on the eastern edge of the tallgrass prairie region of the Northern Great Plains. Common native range plants are bluestems, gramas, Indiangrass, coneflowers, butterfly milkweed, and leadplant, to name a few. The refuge is a major producer of wood ducks, with approximately 2,500 produced annually, and also has been the site of several successful reintroductions of the trumpeter swan. This 3,334-acre refuge surrounded by a sea of corn and soybeans provides vital habitat for a variety of plants and animals.