With high plains in its western panhandle, and wet lowlands in its southeastern corner, Oklahoma is diverse in its terrain and ecology. The Red River forms the southern boundary of the state, and its tributaries drain the southern portion of the state. The Arkansas River and its tributaries drain northern Oklahoma, many interrupted by dams. In fact, Oklahoma has more reservoirs than any other state. The Ozark and Ouachita Mountains occupy the eastern third of the state, about a quarter of the state is forested, with sweet gum, hickory, and maple trees among the dozens of species of trees native to the state. Much of what was once tallgrass prairie is now agricultural land. The abundance of man-made lakes, and the diversity of terrain and habitats—and the fact that it is along the Central Flyway, result in 452 bird species on the Oklahoma checklist.