The Cowlitz has a 2,480 sq mi (4,017.6 sq km) 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 (170 km) in total length, not counting tributaries.
Major tributaries of the Cowlitz River include the Lewis River and the Toutle River, which was overtaken by volcanic mudflows (lahars) during the May 1980 eruption of Mount St. Helens.