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Flathead River, river rising in the MacDonald Range in southeastern British Columbia, Can., and flowing south for 240 miles (385 km) across the Canada–United States boundary into Montana. After passing between the Whitefish Range (west) and Glacier National Park and the Lewis Range (east), it enters Flathead Lake and emerges from the lake to join the Clark Fork near Paradise, Mont. Its chief tributaries are the Middle Fork, which rises near the Continental Divide and...