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. Three Forks is part of the Bozeman, MT Micropolitan Statistical Area of approximately 100,000 people and located thirty miles west of Bozeman.
The three rivers, west to east, were named by Meriwether Lewis in late July 1805 for President Thomas Jefferson, Secretary of State James Madison, and Treasury Secretary Albert Gallatin which was in the times the genesis of a mild controversy and eventually spawned a modern-day geographical controversy—in both cases regarding length comparisons between the Missouri and Mississippi rivers. Today the two confluences are incorporated inside Missouri Headwaters State Park, which is also a U.S. National Historic Landmark.
The Lewis and Clark expedition visited the site on July 28, 1805. Meriwether Lewis in his journal entry wrote:
One consequence of their decision to designate, map and name the Jefferson—the largest— as a separate tributary river, is that today the Mississippi river can arguably be called longer than the Missouri river because extensive re-channelization of the streambed for hydroelectric power projects has shortened the river while the Mississippi Delta has grown lengthening the rival river.
Honored in Three Forks, the Indian woman Sacagawea is best known as the interpreter and guide for the Lewis and Clark Expedition.