The Stillwater River starts in the Absaroka-Beartooth Wilderness south of the Beartooth Mountains in southern Park County, near the state line with Wyoming. It runs northeast, between the Absaroka Range to the west and the Beartooth Mountains to the east, through Custer National Forest, past Nye and Absarokee. It joins the Yellowstone near Columbus, Montana.
This Stillwater is a blue ribbon fishery. Trout are the primary game fish, but Mountain Whitefish are also common. In the river's lower sections, Rainbow Trout and Brown Trout are the most common, but towards the headwaters Yellowstone Cutthroat Trout and Brook Trout show up with increased frequency. In the spring Rainbow Trout from the Yellowstone River enter the Stillwater to spawn, some traveling as far upstream as Nye. In the fall, Brown Trout also enter the river from the Yellowstone to spawn. It is during these seasons that larger fish up to 5 pounds can be more easily found. Despite its misleading name, the Stillwater's most popular stretches contain class II and III whitewater. While float fishing from a raft is a common way to fish the river, it is recommended that only experienced rowers attempt to navigate it.