river in southwestern Montana and northwestern Wyoming, U.S. The Madison River rises in the northwestern corner of Yellowstone National Park at the junction of the Gibbon and Firehole rivers. It flows west through Hebgen Lake (impounded by a dam) into southwestern Montana, then turns north between the Madison Range (to the east) and the Gravelly Range and the Tobacco Root Mountains (to the west) to flow through Ennis Lake (which was created by Madison Dam), Bear Trap Canyon, and a section of the Lee Metcalf Wilderness. After a course of 183 miles (294 km), the Madison joins the Gallatin (Gallatin River) and Jefferson (Jefferson River) rivers just northeast of Three Forks to form the headwaters of the Missouri River.