Wahkiakum County is a county located in the U.S. state of Washington. As of the 2020 census, the population was 4,422, making it one of the least populous counties in Washington. The county seat and only incorporated town is Cathlamet. The county was formed out of Cowlitz County in April 1854 and is named for Chief Wahkiakum of the Chinook, who is buried in the Pioneer Cemetery in Cathlamet.

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.