 The Rocky Mountains of southwestern Montana at the headwaters of the Missouri River first rose in the Laramide <unk> , a mountain @-@ building episode that occurred from around 70 to 45 million years ago ( the end of the <unk> through the early Cenozoic ) . This <unk> <unk> Cretaceous rocks along the western side of the Western Interior <unk> , a vast shallow sea that stretched from the Arctic Ocean to the Gulf of Mexico , and deposited the sediments that now <unk> much of the drainage basin of the Missouri River . This Laramide <unk> caused the sea to retreat and laid the framework for a vast drainage system of rivers flowing from the Rocky and Appalachian Mountains , the predecessor of the modern @-@ day Mississippi watershed . The Laramide <unk> is essential to modern Missouri River hydrology , as snow and ice melt from the Rockies provide the majority of the flow in the Missouri and its tributaries . 
