Susanville (formerly known as Rooptown) is the county seat of Lassen County, California, United States. Susanville is located on the Susan River in southern part of the county, at an elevation of 4,186 feet (1,276 m). The population was 17,974 at the 2010 census, up from 13,541 at the 2000 census. Susanville, a former logging and mining town, is the site of the High Desert State Prison, California (not to be confused with High Desert State Prison, Nevada) and California Correctional Center.