There are 7 US Post Offices in Benton County serving a total of 111,779 residents equating to 15,968 residents per post office. It's estimated that approximately 178,412 packages are handled across the 7 county post offices annually. Benton County is a county located in the south-central portion of the U.S. state of Washington. As of the 2010 census, its population was 175,177. The county seat is Prosser, and its largest city is Kennewick. The Columbia River demarcates the north, south, and east boundaries of the county. Benton County was created from what were then larger versions of Klickitat County and Yakima County on 8 March 1905. and was named after U.S. Senator from Missouri (1821-1851) and later U.S. Representative (1853-1855) Thomas Hart Benton."