There are 9 US Post Offices in Cowlitz County serving a total of 91,480 residents equating to 10,164 residents per post office. It's estimated that approximately 146,012 packages are handled across the 9 county post offices annually. 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 \u201criver of shifting sands\u201d or \u201ccapturing the medicine spirit.\u201d Cowlitz comprises the Longview, WA Metropolitan Statistical Area, which is also included in the Portland-Vancouver-Salem, OR-WA Combined Statistical Area."