Cowlitz County is a county located in the U.S. state of Washington. As of the 2010 census, its population was 102,410. The county seat is Kelso, and its largest city is Longview. The county was formed on April 21, 1854. 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.”