River Falls is a city in Pierce and St. Croix counties in the U.S. state of Wisconsin. It is located mostly within the Town of River Falls in Pierce County; a small portion lies within the Town of Kinnickinnic in St. Croix County. River Falls is the most populous city in Pierce county. The population was 15,000 at the 2010 census, with 11,851 residing in Pierce County, and 3,149 in St. Croix County. It is part of the Minneapolis-St.