Horicon Marsh encompasses both Horicon National Wildlife Refuge and Horicon Marsh State Wildlife Area. The marsh was formed in the late Pleistocene as a shallow post-glacial lake when the retreating ice created a recessional moraine at the south end. This impounded the glacial meltwaters to create the lake, which eventually drained as its outlet was eroded. The surrounding uplands contain the highest concentration of glacial drumlins in the world. The marsh itself is a wide expanse measuring 13.5 miles long by 3-5 miles wide. The marsh was ditched and drained in the early 1900?s and was later restored, resulting in altered hydrology and vegetation composition. Currently, it consists primarily of cattail marsh and open water, with stands of burreed, phragmites, reed canary grass, islands of brush and hardwood forest. The uplands contain many woodlots in an agricultural landscape.
Horicon is the largest freshwater cattail marsh in the United States. It supports very high numbers of migrating waterfowl, including some 250,000 Canada Geese and 100,000 ducks during fall migration. Significant numbers of shorebirds also use Horicon as a migratory stopover when appropriate habitat conditions are available. Horicon hosts the largest breeding population of Redhead east of the Mississippi River. It also supports significant breeding populations of waterbirds such as Least Bittern, American Bittern, Great Blue Heron, Great Egret, Black-crowned Night-Heron, American White Pelican, Forster?s Tern, and Black Tern. Grassland habitat in the surrounding uplands supports some upland-nesting marsh birds such as Mallard and Blue-winged Teal, as well as grassland birds such as Bobolink and Dickcissel. The potential exists to restore larger blocks of grassland to provide suitable habitat for additional grassland species.
Horicon Marsh was designated a Wetland of International Importance under the Ramsar Convention in 1991 and a Globally Important Bird Area in 2001. Invasive species, including carp, wild parsnip, reed canary grass, phragmities, buckthorn, honeysuckle, and purple loosestrife are a problem throughout the marsh. Control efforts are ongoing. Water quality is affected from industrial and agricultural pollution sources. Potential exists to increase the extent and quality of grassland habitats for nesting ducks and other grassland birds. Prairie and oak savanna restoration efforts are underway and should continue. Monitoring efforts, particularly for secretive marsh birds and grassland birds, should be continued and expanded.