Madison, river, 183 mi (295 km) long, rising in Yellowstone National Park, NW Wyo., and flowing W then N through SW Montana to join the Jefferson and Gallatin rivers at the Three Forks of the Missouri. It is impounded by Hebgen Dam in its upper course and by Madison Dam, a power facility, at midcourse. The river is used for irrigation. Earthquake Lake was formed in 1959.
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