Sioux City is a city in Woodbury and Plymouth counties in the northwestern part of the U.S. state of Iowa. The population was 82,684 in the 2010 census, which makes it the fourth largest city in Iowa. The bulk of the city is in Woodbury County, of which it is the county seat, though a small portion is in Plymouth County. Sioux City is located at the navigational head of the Missouri River. Sioux City is the primary city of the five-county Sioux City, IA–NE–SD Metropolitan Statistical Area (MSA), with a population of 168,825 in 2010 and a slight increase to an estimated 168,921 in 2012.The Sioux City–Vermillion, IA–NE–SD Combined Statistical Area had a population of 182,675 as of 2010 and has grown to an estimated population of 183,052 as of 2012.