Silver Falls state park, Oregon

website, Silver Falls State Park:

"Where else can you walk behind a waterfall? Check out the famous South Falls and see what a 177-foot curtain of water looks like from behind. It’s part of the Trail of Ten Falls, a spectacular, nationally recognized hiking trail that weaves through a dense forested landscape. The trail passes a series of breathtaking waterfalls along a rocky canyon, and descends to a winding creek at the forest floor. This 7.2 mile loop is considered to be a moderate hike, with an overall elevation change of 800 feet. Several connecting trails with separate access points make shorter routes."
---about ninety minutes southeast from Portland