Tilton RiverThe Tilton River is a tributary of the Cowlitz River, in the U.S. state of Washington. Named for the first territorial surveyor for Washington Territory James Tilton, it flows for about, entirely within Lewis County.CourseThe Tilton River originates in the Cascade Range just north of Mount St. Helens and southwest of Mount Rainier. It flows south and west, joining the Cowlitz River in Lake Mayfield, near Mossyrock.