The topography of Iowa consists of a gently rolling plain that slopes from the highest point of 1,670 ft (509 m) in the northwest to the lowest point of 480 ft (146 m) in the southeast at the mouth of the Des Moines River. About two-thirds of the state lies between 800 ft (244 m) and 1,400 ft (427 m) above sea level; the mean elevation of land is 1,100 ft (336 m).
Supremely well suited for agriculture, Iowa has the richest and deepest topsoil in the US and an excellent watershed. Approximately two-thirds of the state's area is drained by the Mississippi River, which forms the entire eastern boundary, and its tributaries. The western part of the state is drained by the Missouri River and its tributaries. Iowa has 13 natural lakes. The largest are Spirit Lake (9 mi/14 km long) and West Okoboji Lake (6 mi/10 km long), both near the state's northwest border.
The Iowa glacial plain was formed by five different glaciers. The last glacier, which covered about one-fifth of the state's area, retreated from the north-central region some 10,000 years ago, leaving the topsoil as its legacy. Glacial drift formed the small lakes in the north. The oldest rock outcropping, located in the state's northwest corner, is about 1 billion years old.