The Cowlitz River is a river in the state of Washington in the United States, a tributary of the Columbia River. Its tributaries drain a large region including the slopes of Mount Rainier, Mount Adams, and Mount St. Helens.The Cowlitz has a 2,586-square-mile (6,698 km2) drainage basin, located between the Cascade Range in eastern Lewis County, Washington and the cities of Kelso and Longview. The river is roughly 105 miles (169 km) long, not counting tributaries.