Rock Falls is a city in Cerro Gordo County, Iowa, United States, along the Shell Rock River. The population was 150 at the time of the 2020 census. It is part of the Mason City Micropolitan Statistic…Rock Falls is a city in Cerro Gordo County, Iowa, United States, along the Shell Rock River. The population was 150 at the time of the 2020 census. It is part of the Mason City Micropolitan Statistical Area.