 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 . Flowing west then north , it runs first in Hell Roaring Creek , then west into the Red Rock ; swings northeast to become the Beaverhead , it finally joins with the Big Hole to form the Jefferson . The Firehole River originates at Madison Lake in Wyoming 's Yellowstone National Park and joins with the Gibbon 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 . 
 From the Rocky Mountains of Montana and Wyoming , three streams rise to form the headwaters of the Missouri River . The longest <a href='uninflect' title='1'><mark>begins</mark></a> near Brower <a href='null_genetive' title='2'><mark>'s</mark></a>Spring , 9 @,@ 100 feet ( 2 @,@ 800 m ) above sea level , on the southeastern slopes of Mount Jefferson in the Centennial Mountains . Flowing west then north , it <a href='uninflect' title='3'><mark>runs</mark></a> first in Hell Roaring Creek , then west into the Red Rock ; swings northeast to become the Beaverhead , it finally <a href='uninflect' title='4'><mark>joins</mark></a> with the Big Hole to form the Jefferson . The Firehole River originates at Madison Lake in Wyoming <a href='null_genetive' title='5'><mark>'s</mark></a>Yellowstone National Park and <a href='uninflect' title='6'><mark>joins</mark></a> with the Gibbon to form the Madison , while the Gallatin River <a href='uninflect' title='7'><mark>rises</mark></a> out of Gallatin Lake , also in the national park . These two streams then flow north and northwest into Montana . 
From da Rocky Mountains of Montana and Wyoming , three streams rise to form the headwaters of the Missouri River . Tdalongest begin near Brower  Spring , 9 @,@ 100 feet ( 2 @,@ 800 m ) above sea level , on thdaoutheastern slopes of Mount Jefferson in the Centennial Mountains . Flowing west then north , it run first in Hell Roaring Creek , then west into thedad Rock ; swings northeast to become the Beaverhead , it finally join w the dag Hole to form the Jdaerson . The Fidaole River originates at Madison Lake in Wyoming  Yellowstone National Park and join with the Gibda to form the Madida , while the Gallada River rise out of Gallatin Lake , also in the national park . These two streams then flow north and northwest into Montana .