The Boulder River originates high in the Absaroka-Beartooth Wilderness just north of Yellowstone Park and flows north through Park and Sweet Grass Counties before joining the Yellowstone River near Big Timber. This freestone stream tumbles through some of Montana's most spectacular mountain scenery, with the Beartooth Mountains to the east and the Absarokas to the west. The river's exceptional clarity—often described as aquarium-like—reveals a substrate dominated by boulders in every shape, size, and color, giving the river its fitting name. This Blue Ribbon trout stream offers outstanding pocket water fishing throughout its 35-mile fishable length. While moderate fishing pressure keeps the experience relatively uncrowded, access challenges due to extensive private land ownership in the lower sections and difficult wading conditions naturally limit angler traffic. The Boulder River represents one of Montana's hidden gems, offering skilled anglers willing to work for their fish an exceptional wilderness fishing experience.