Cache River State Natural Area is situated in southernmost Illinois within a floodplain carved long ago by glacial floodwater of the Ohio River. When the Ohio River adopted its present course, it left the Cache River to meander across rich and vast wetlands.
Despite intensive efforts to convert land along the Cache River to cropland, the land that today makes up the Cache River State Natural Area has managed to hold onto some of the highest quality aquatic and terrestrial natural communities remaining in Illinois. Wetlands within this area are so important to migratory waterfowl and shorebirds that in 1996 the RAMSAR Convention collectively designated them a Wetland of International Importance, only the 19th wetland in the United States to receive the distinction. It is within southern Illinois that north meets south and east meets west. With its diversity of soils, bedrock and landforms, the Cache River Valley contains four distinct ecological regions. Its hodgepodge of ecological factors has resulted in a collage of natural communities, including the bald cypress-tupelo swamp, each with its own unique assemblage of physical attributes, plants and animals.
This site was chosen as an IBA because it met the criteria for breeding for the Yellow-crowned Night-heron, Brown Creeper, Prothonotary Warbler, and the Prairie Warbler as well as Wading Birds. More than 13,000 acres of prime wetland and forest habitat in the Cache River State Natural Area provide homes for breeding and migratory birds of nearly all species of birds found in Illinois. These include waders such as the Yellow-crowned Night-Heron, which breeds along backwater sloughs and swamps, as well as Great Egrets and Great Blue Herons. A variety of raptors including Missisipi Kite, Red-shouldered Hawk and Broad-winged Hawk nest within this complex. Copious warbler species and other songbirds migrate through or remain to nest. Nesting songbirds include Red-headed Woodpecker, Brown Creeper, Kentucky Warbler, Worm-eating Warbler, Prothonotary Warbler, Cerulean Warbler, Prairie Warbler, Black-and-White Warbler, Northern Parula, American Redstart, Louisiana Waterthrush, Hooded Warbler, Ovenbird, Acadian Flycatcher, Blue Grosbeak, White-eyed Vireo, Dickcissel, Grasshopper Sparrow and Henslow's Sparrow. Pileated Woodpeckers are also common breeders as well as Wild Turkey and Fish Crow. Northern Harriers, Short-eared Owls and Red-headed Woodpeckers winter in the Cache River region.
The swamps of Cache River SNA are in low ridges of the Lesser Shawnee Hills. At the base of these hills, bottomland hardwood forests dominated by overcup oak, pin oak, cherrybark oak and sweetgum give way to red oak, white oak and shagbark hickory. Barrens occur on the highest ridge tops where soils are thin and bedrock is exposed. These sites are dominated by small post oak and blackjack oak trees scattered about open expanses of land dominated by grasses and forbs.