Three Forks is a city in Gallatin County, Montana, United States and is located within the watershed valley system of both the Missouri and Mississippi rivers drainage basins — and is historically considered the birthplace or start of the Missouri River. The population was 1,869 at the 2010 census. The city of Three Forks is named so because it lies geographically near the point, in nearby Missouri Headwaters State Park, where the Jefferson, Madison, and Gallatin Rivers converge to form the Missouri River — the longest single river in North America, as well as the major portion of the Missouri-Mississippi River System from the headwaters near Three Forks to its discharge into the Gulf of Mexico.