The Flathead River of northwest Montana rises via three primary forks. The North Fork heads in the mountains of southeast British Columbia and then flows south along the western edge of Glacier National Park. The Middle Fork rises in the Rocky Mountains, northwest of Great Falls, winding northwest and then westward to join the North Fork. The South Fork also rises in the Rockies, more directly west of Great Falls, and flows NNW, where it enters Hungry Horse Reservoir before merging with the combined North and Middle Forks. The primary channel of the Flathead River then enters the Rocky Mountain Trench, a broad valley formed by downwarping of the crust as mountains rose to its east and later occupied by Pleistocene glaciers. Flowing southwestward and then southward, the river passes Kalispell, Montana, and enters Flathead Lake; the Stillwater, Whitefish and Swan Rivers also feed the lake.
The largest natural freshwater lake (by area) in the western Lower 48, Flathead Lake initially formed from glacial meltwater behind a terminal moraine that was deposited late in the Pleistocene; the lake valley was also inundated by Glacial Lake Missoula as it expanded and retreated 30-15,000 years ago (see my blog on 4-2-12). Exiting the southwest corner of its lake, the Flathead River snakes southward across a landscape of plateaus and ridges before flowing westward through a rugged canyon to join the Clark Fork River.
The upper forks of the Flathead have all been designated National Wild & Scenic Rivers and are among the most remote and least disturbed streams in our country. Nevertheless, the North Fork faced possible contamination from proposed coal mining and gas production in southeastern British Columbia over the past few decades; fortunately, an agreement between the U.S. and Canada has, for now, blocked that "development."