Cowlitz County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 110,730. The county seat is Kelso, and its largest city is Longview. The county was formed in April 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.' Cowlitz comprises the Longview, WA Metropolitan statistical area, which is also included in the Portland-Vancouver-Salem, OR-WA Combined statistical area.

Lewis County is a county in the U.S. state of Washington. As of the 2020 census, the county's population was 82,149. The county seat is Chehalis, and its largest city is Centralia. Lewis County comprises the Centralia, WA Micropolitan Statistical Area, which is also included in the Seattle-Tacoma, WA Combined Statistical Area.