The Flathead River, in the northwestern part of Montana, originates in the Rocky Mountains near Glacier National Park and flows southwest into Flathead Lake, then after a journey of 158 miles (254 km), empties into the Clark Fork. The river is part of the Columbia River drainage basin, as the Clark Fork is a tributary of the Pend Oreille River, a Columbia River tributary. The river is a Class I river in Montana for purposes of recreational access. All of the headwaters forks are entirely or in part designated National Wild and Scenic Rivers.

From Wikipedia.