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 portion 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 named after the Des Moines River, which may have been adapted from the French Rivière des Moines, meaning "River of the Monks". The city's population was 203,433 as of the 2010 census. The five-county metropolitan area is ranked 91st in terms of population in the United States with 599,789 residents according to the 2013 estimate by the United States Census Bureau.