Missouri Headwaters State Park near Three Forks, Montana, preserves the confluence where the Jefferson and Madison join and the Missouri River officially begins, a defining landscape in the American West. Long used as a campsite by fur trappers, it is especially significant for the Lewis and Clark Expedition, which camped here in 1805, explored the three branches, and named the Jefferson, Madison, and Gallatin rivers before continuing up the Jefferson and returning in 1806. Its importance led to preservation efforts in the 1960s and National Historic Landmark designation in 1960. The site also stands at the center of enduring debates over the Missouri’s true source and total length.