White River is a city in and the county seat of Mellette County. South Dakota, United States. The population was 533 at the 2020 census. …White River is a city in and the county seat of Mellette County. South Dakota, United States. The population was 533 at the 2020 census. White River was founded as the seat of the newly formed Mellette County in 1911. The town was named after the White River.