Beyond being one of the most wild and scenic rivers in Oregon, the Metolius is one of the largest spring-fed rivers in the United States, with a unique geological story.
Unlike most of the rivers of the Pacific Northwest, which originate high on mountain tops and are sustained by snow and glacial melting, the Metolius River begins at a calm and subtle spring far from the towering peaks of the Cascade Range. Emerging as a humble trickle, the spring is tucked away at the base of Black Butte's northern slope. While this is where the river begins, its ultimate source isn't actually all that different than most nearby rivers, which is where the story gets interesting.
About 4 million years ago the crest of the previous generation of Cascade Mountains sank thousands of feet, forming a giant depression. Since then, constant volcanic activity has given rise to the current generation of peaks such as Mount Washington, Three Fingered Jack and Mount Jefferson. The lava formations from more recent volcanic activity have nearly filled this once giant depression. Green Ridge still stands as the eastern fault line to the depression, and the Metolius River flows down the valley created by this fault line. Black Butte, now a long-extinct volcano, arose right on top of this eastern fault, burying the Metolius River.
So, although the river appears out of nowhere, the rest of its drainage basin is simply on the other south side of the butte, making its true source high in the Cascades. Black Butte effectively created a sprawling dam, hence the numerous swampy meadows on the butte's south side, such as those around Black Butte Ranch.