There are 6 US Post Offices in Richland County serving a total of 10,480 residents equating to 1,747 residents per post office. It's estimated that approximately 16,727 packages are handled across the 6 county post offices annually. Richland County is a county located in the State of Montana. As of the 2010 census, the population was 9,746. Its county seat is Sidney. Richland County was created by the Montana Legislature in 1914 from part of Dawson County. An early proposed name for the county was Gate, but Richland was decided upon instead as a way to entice new settlers."