The eastern quarter of the state is dominated by the Cumberland Plateau, on the western border of the Appalachians. At its western edge, the plateau meets the uplands of the Lexington Plain (known as the Bluegrass region) to the north and the hilly Pennyroyal to the south. These two regions, which together make up nearly half the state's area, are separated by a narrow curving plain known as the Knobs because of the shapes of its eroded hills. The most level area of the state consists of the western coalfields bounded by the Pennyroyal to the east and the Ohio River to the north. In the far west are the coastal plains of the Mississippi River; this region is commonly known as the Purchase, having been purchased from the Chickasaw Indians.
The highest point in Kentucky is Black Mountain on the southeastern boundary in Harlan County, at 4,139 ft (2,162 m). The lowest point is 257 ft (78 m), along the Mississippi River in Fulton County. The state's mean altitude is 750 ft (229 m).
The only large lakes in Kentucky are artificial. The biggest is Cumberland Lake (79 sq mi/205 sq km); Kentucky Lake, Lake Barkley, and Dale Hollow Lake straddle the border with Tennessee.
Including the Ohio and Mississippi rivers on its borders and the tributaries of the Ohio, Kentucky claims at least 3,000 mi (4,800 km) of navigable rivers—sometimes said to have more water than any other state except Alaska. Among the most important of Kentucky's rivers are the Kentucky, 259 mi (417 km); the Cumberland, partly in Tennessee; the Tennessee, also in Tennessee and Alabama; and the Big Sandy, Green, Licking, and Tradewater rivers. All, except for a portion of the Cumberland, flow northwest into the Ohio and thence to the Mississippi. Completion in 1985 of the Tennessee-Tombigbee Waterway, linking the Tennessee and Tombigbee rivers in Alabama, gave Kentucky's Appalachian coalfields direct water access to the Gulf of Mexico for the first time.
Drainage through porous limestone rock has honeycombed much of the Pennyroyal with underground passages, the best known of which is Mammoth Cave, now a national park. The Cumberland Falls, 92 ft (28 m) high and 100 ft (30 m) wide, are located in Whitely County.