River is a tributary of the Yellowstone River, approximately 60 mi (96 km) long, in south central Montana
in the United States
. It is one of two rivers named the Boulder River in Montana.It rises in the Gallatin National Forest
in the Absaroka Range in southern Park County
. It flows north through mountainous canyons, a cataract under a natural bridge, and northwest in a widening valley past McLeod. It joins the Yellowstone at Big Timber
The river is a popular destination for fly fishing. Other attractions in the valley include several church camps, a guest ranch, and several USFS campgrounds. The Forest Service also maintains several guard stations, trailheads, and picnic areas in the valley.