The Gardner River (also known as the Gardiner River) is a tributary of the Yellowstone River, approximately 25 miles (40 km) long, in northwestern Wyoming and south central Montana in the United States. The entire river is located within Yellowstone National Park. It rises on the slope of Joseph Peak, Gallatin Range in the northwestern part of the park, and winds southeast through Gardner’s Hole, a broad subalpine basin which is a popular trout fishing location. Angling on the Gardner is governed by Yellowstone National Park fishing regulations. After merging with Panther Creek, Indian Creek and Obsidian Creek, it then turns north and flows through a steep canyon where it cuts through a basaltic flow from approximately 500,000 years ago known as Sheepeater Cliffs. Below Sheepeater, Glen Creek out of Golden Gate Canyon and Lava Creek out of Lava Creek Canyon join the Gardner near Mammoth Hot Springs. The river crosses the 45th parallel in Gardner Canyon and is also home to a popular hot spring known as The Boiling River. The river continues north through Gardner Canyon and empties into the Yellowstone near Gardiner, Montana.