Cowlitz County is a county located in the U.S. state of Washington. As of the 2010 census, its population was 102,410. It forms the Longview, Washington, Metropolitan Statistical Area which encompasses all of Cowlitz County. The county seat is at Kelso, and its largest city is Longview. Its name derives from the anglicized version of the Cowlitz Indian term, Cow-e-liske, meaning either “river of shifting sands” or “capturing the medicine spirit.” It was formed on April 21, 1854.