Arkansas Post National Memorial totals 758 acres and includes the Memorial Unit (389 acres) and the Osotouy Unit (361 acres). The two units are separated by the Arkansas River and land. The Memorial Unit, a peninsula into the Arkansas River, was first designated to preserve and commemorate the first European settlement in the lower Mississippi Valley. The Osotouy Unit is located along Lake Dumond and a bayou. It was added to the park to preserve the mounds and cemetery associated with pre-Columbian and early European settlement.
The park is at the northern edge of the Gulf coastal plain. Native vegetation ranges from prairie grasses and bottomland hardwood forests, to wetland marshes near the bayous and river. Plant communities include bottomland and upland forests, swamps, fresh water marshes, canebrakes, isolated prairie relics, and manicured lawns. Trees cover almost 80% of the park and over 270 vascular plant species have been identified within its borders. Eastern North American forest types, such as oak, hickory, and black walnut can be found mixed with bottomland sweetgum-oak associates as the park topography changes.