Des Moines is the capital and the most populous city in the U.S. state of Iowa. It is also the county seat of Polk County. A small part of the city extends into Warren County. It was incorporated on September 22, 1851, as Fort Des Moines, which was shortened to "Des Moines" in 1857. It is located on, and named after, the Des Moines River, which likely was adapted from the early French name, Rivière des Moines, meaning "River of the Monks". The city's population was 216,853 as of the 2018 population estimate. The five-county metropolitan area is ranked 89th in terms of population in the United States with 655,409 residents according to the 2018 estimate by the United States Census Bureau, and is the largest metropolitan area fully located within the state. A portion of the larger Omaha, Nebraska, metropolitan area extends into three counties of southwest Iowa.