There are 3 US Post Offices in Wahkiakum County serving a total of 3,470 residents equating to 1,157 residents per post office. It's estimated that approximately 5,539 packages are handled across the 3 county post offices annually. Wahkiakum County /w\u0259\u02c8ka\u026a.\u0259k\u028cm/ is a county located in the U.S. state of Washington. As of the 2010 census, the population was 3,978, making it the second-least populous county in Washington. The county seat and only incorporated town is Cathlamet. The county was formed out of Cowlitz County on April 25, 1854 and is named for Chief Wahkiakum (\"Tall Timber\") of the Chinook. The County operates the Wahkiakum County Ferry, which connects Cathlamet to Westport, Oregon, across the Columbia River."