Horicon NWR is located on the west branch of the Rock River in southeastern Wisconsin. The Refuge manages the northern two thirds and the Department of Natural Resources manages the southern one third of the Horicon Marsh.
At 32,000 acres, Horicon Marsh is the largest freshwater cattail marsh in the United States. Horicon Marsh is bounded on the east by a sharply rising ridge of the Niagara escarpment which rises approximately 250 feet above the marsh to an elevation of 1,100 feet. The land to the west of the marsh rises slowly and is dotted with many small potholes and several shallow lakes.
In 1990, Horicon Marsh was designated a "Wetland of International Importance" by the Ramsar Convention. Up to 300,000 Canada geese stage on the Refuge in the fall. The area is also a mecca for ducks, cranes, herons, and shorebirds.