The Willamette Floodplain consists of 713 acre of natural grassland, near the Willamette River, that was made a National Natural Landmark in May 1987. The floodplain is within the William L. Finley National Wildlife Refuge and located about 10 mi south of Corvallis in Benton County, Oregon. It is the largest remaining native unplowed example of bottomland interior valley grassland in the North Pacific Border natural region. It was classified as a natural landmark because such grassland and shrubland areas are exceedingly rare, with most having been cultivated or turned into pastureland.