The Boston Mountains (an extension of the Ozark Plateau, sometimes called the Ozark Mountains) in the northwest and the Ouachita Mountains in the west-central region not only constitute Arkansas's major uplands but also are the only mountain chains between the Appalachians and the Rockies. Aside from the wide valley of the Arkansas River, which separates the two chains, the Arkansas lowlands belong to two physiographic regions: the Mississippi Alluvial Plain and the Gulf Coastal Plain. The highest elevation in Arkansas, at 2,753 ft (840 m), is Magazine Mountain, standing north of the Ouachitas in the Arkansas River Valley. The state's lowest point, at 55 feet (17 meters), is on the Ouachita River in south-central Arkansas.
Arkansas's largest lake is the artificial Lake Ouachita, covering 63 sq mi (163 sq km); Lake Chicot, in southeastern Arkansas, and oxbow of the Mississippi River, is the state's largest natural lake, with a length of 18 mi (29 km). Bull Shoals Lake, occupying 71 sq mi (184 sq km), is shared with Missouri. Principal rivers include the Mississippi, forming most of the eastern boundary; the Arkansas, beginning in Colorado and flowing 1,450 mi (2,334 km) through Kansas and Oklahoma and across central Arkansas to the Mississippi; and the Red, White, Ouachita, and St. Francis rivers, all of which likewise drain south and southeast into the Mississippi. Numerous springs are found in Arkansas, of which the best known are Mammoth Springs, near the Missouri border, one of the largest in the world, with a flow rate averaging nine million gal (34 million l) an hour, and Hot Springs in the Ouachitas.
Crowley's Ridge, a unique strip of hills formed by sedimentary deposits and windblown sand, lies west of and parallel to the St. Francis River for about 180 mi (290 km). The ridge is rich in fossils and has an unusual diversity of plant life.