Hawarden is a city in Sioux County, Iowa, United States. The population was 2,700 at the time of the 2020 census. It is located on the Big Sio…Hawarden is a city in Sioux County, Iowa, United States. The population was 2,700 at the time of the 2020 census. It is located on the Big Sioux River. Hawarden was platted in 1882. It was named for Hawarden Castle, the Welsh home of statesman William Gladstone. Hawarden was incorporated as a city on March 18, 1887.