- Kelso is a city in Cowlitz County, Washington, United States. It is located in the southwestern part of the state and has a population of approximately 12,000 people. Kelso is situated along the Cowlitz River and is known for its scenic beauty and outdoor recreational opportunities. The city is also home to the Three Rivers Mall, which serves as a major shopping destination for the surrounding area. Kelso is located about 50 miles north of Portland, Oregon, making it a popular commuter town for those working in the Portland metropolitan area.