 As one of the continent 's most significant river systems , the Missouri 's drainage basin borders on many other major watersheds of the United States and Canada . The Continental Divide , running along the spine of the Rocky Mountains , forms most of the western border of the Missouri watershed . The Clark Fork and Snake River , both part of the Columbia River basin , drain the area west of the Rockies in Montana , Idaho and western Wyoming . The Columbia , Missouri and Colorado River watersheds meet at Three Waters Mountain in Wyoming 's Wind River Range . South of there , the Missouri basin is bordered on the west by the drainage of the Green River , a tributary of the Colorado , then on the south by the mainstem of the Colorado . Both the Colorado and Columbia Rivers flow to the Pacific Ocean . However , a large <unk> drainage called the Great Divide Basin exists between the Missouri and Green watersheds in western Wyoming . This area is sometimes counted as part of the Missouri River watershed , even though its waters do not flow to either side of the Continental Divide . 
