The New River Mountains are a small 14-mi long, mountain range in central Arizona, and on the north border of the Phoenix valley; the range is located on the southwest perimeter of the Arizona transition zone. The range is a sub-part of landforms extending south from the Black Hills of Yavapai County. Rivers and canyons border east and west; the sout…The New River Mountains are a small 14-mi long, mountain range in central Arizona, and on the north border of the Phoenix valley; the range is located on the southwest perimeter of the Arizona transition zone. The range is a sub-part of landforms extending south from the Black Hills of Yavapai County. Rivers and canyons border east and west; the south of the range is located in Maricopa County, the location of much of the Phoenix metropolitan area.