Forest River is a city in Walsh County, North Dakota, United States. The population was 109 at the 2020 census. Forest River was founded o…Forest River is a city in Walsh County, North Dakota, United States. The population was 109 at the 2020 census. Forest River was founded on July 13, 1887. According to the United States Census Bureau, the city has a total area of 0.50 square miles, all land.