Union Slough is all that remains of a pre-glacial riverbed. Actually, the area is a connection point, or union, between two watersheds - the Blue Earth River of Minnesota and the east fork of the Des Moines River. Before European settlement, the area was so level that the wind determined which way the slough flowed. Established in 1938 as a migratory waterfowl refuge, Union Slough National Wildlife Refuge provides critical habitat for native prairie and wetland wildlife in a landscape dominated by intensive agriculture.