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is a city in Monona County, Iowa, United States. The population was 3,091 at the 2000 census. It is the county seat of Monona County. It is the largest town on the Iowa side of the Missouri River between Council Bluffs and Sioux City.
The city's main claim to fame is that it has the widest main street in the United States - about wide.
According to the United States Census Bureau, the city has a total area of 4.9 square miles (12.7 km²), all of it land.
Onawa is located in the Loess Hills region of western Iowa, a unique geological and environmental area. Nearby are such natural areas as Lewis & Clark State Park, Preparation Canyon State Park and the Loess Hills State Forest.