The source of the Missouri River is located at Brower's Spring at the source of the tributary known as the Jefferson River. Brower's Spring was discovered in the Centennial Mountains by Jacob V. Brower in the 1890s.
The source of a river is defined as the point on the river that is the furthest from its end. However, when Lewis and Clark reached the area that is now the Missouri Headwaters State Park, Lewis decided that none of the three forks was superior to the others and said that the location at three forks was the source of the Missouri. Therefore, determining the source of the Missouri required the Lewis and Clark Trail Heritage Foundation to determine which of the three tributaries is the longest.