Cowlitz County was organized in 1854 from Lewis County and was one of the first counties created by Washington's First Territorial Legislature. It is named after the river that Lewis and Clark called Cow-e-liske or "river of the shifting sand."

Klickitat County was founded in 1859 from part of Skamania County. It was named after the Klickitat Indian Tribe and the word means "robber or thief." Klickitat County bases its economy on food processing, agriculture and wood products.

Pacific County was organized in 1851 by the Oregon Territorial Legislature from Lewis County. It is named for the Pacific Ocean. Pacific County's economy is based on wood products, agriculture and fishing.

Skamania County was founded in 1854 from Clark County. Its named means "swift water." The county is bisected north and south by the Cascade Mountains and contains Mount St. Helens, the most active volcano in the continental United States.

Wahkiakum County was founded in 1854 from Lewis County. Its named is an Indian word meaning "tall timber." The county has depended on forest products and dairying as its primary economic pursuits.