 From the Rocky Mountains of Montana and Wyoming , three streams rise to form the headwaters of the Missouri River . The longest begins near Brower 's Spring , 9 @,@ 100 feet ( 2 @,@ 800 m ) above sea level , on the southeastern slopes of Mount Jefferson in the Centennial Mountains . <unk> west then north , it runs first in Hell <unk> Creek , then west into the Red Rock ; swings northeast to become the <unk> , it finally joins with the Big Hole to form the Jefferson . The <unk> River originates at Madison Lake in Wyoming 's Yellowstone National Park and joins with the <unk> to form the Madison , while the Gallatin River rises out of Gallatin Lake , also in the national park . These two streams then flow north and northwest into Montana . 
