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.