Nine Mile Island is located within an extensive river ecosystem that includes the Chippewa River and Nine Mile Slough and features two high quality native plant communities - oak barrens and floodplain forest. Most of the island is an extensive floodplain forest of silver maple, river birch, green ash, swamp white oak, elms, hackberry, and yellow bud hickory. Understory species include hop hornbeam, common winterberry and herbs such as cut-leaved coneflower, fox, bur and swollen sedges. On the island's northeast corner the sandy, gravelly soils support and excellent oak barrens maintained through the scouring action of floods, and other disturbances, such as fire and grazing. Canopy trees are mostly Hill's oak along with a number of bur and red oaks. There are also a few scattered red cedar and white pine. In some areas with 70-80% shading, the groundlayer still harbors an excellent and diverse prairie component including cream and white wild indigo, stiff goldenrod, bush-clover, rough blazing-star, prairie thistle, whorled milkweed, and asters. Grasses are dominated by big and little blue-stem with lesser amounts of Indian grass, needle grass, June grass, prairie cord grass, and three species of drop-seed. The area has been identified as having an exceptionally diverse fauna with a number of rare species, including three freshwater mussel species, 9 species of fish, and numerous plants and other animals that prefer unfragmented floodplain forest as habitat. Nine Mile Island is owned by the DNR and was designated a State Natural Area in 1990.