There are 2 US Post Offices in Columbia County serving a total of 3,891 residents equating to 1,946 residents per post office. It's estimated that approximately 6,210 packages are handled across the 2 county post offices annually. Columbia County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 4,078, making it the third-least populous county in Washington. The county seat and largest city is Dayton. The county was created out of Walla Walla County on November 11, 1875 and is named after the Columbia River. Columbia County is included in the Walla Walla, WA Metropolitan Statistical Area."