Once a free-flowing river system, Big Lake NWR was changed to a lake / swamp ecosystem by the New Madrid earthquakes of 1811-12. Big Lake today consists primarily of wooded swamps and open water. The lake is shallow with an average depth of three feet and is bordered by a virgin cypress-tupelo swamp with some black willow and buttonbush. Smartweed, American lotus and water lily characterize the herbaceous wetland plants. Tree species on higher ground include cottonwood, green ash, hackberry, red maple, sycamore, river birch and a variety of oaks.
Due to the fact that Big Lake NWR is an oasis of bottomland hardwood in an agriculturally developed area, 6,400 acres are designated as a National Natural Landmark and 2,100 acres of the Natural Landmark are included in the Wilderness Preservation System.