Rainbow Falls State Park is a 129-acre recreation area on the Chehalis River in southwest Washington, known for its small basalt waterfall, riverfront camping, trails, and remnants of old-growth forest. Its landscape reflects deep geological change, from an ancient inland sea to lava flows that formed the rock over which the river now runs. Long before it became a state park, Upper Chehalis people used the site for lamprey fishing. Acquired by the state in 1933, it was built by Civilian Conservation Corps Company 1633 and completed in 1935. Repeated floods reshaped the falls and destroyed its famous footbridge, but restoration preserved it as a popular destination for outdoor recreation.