Evolution of the Upper Missouri River drainage in southwest Montana

This video presents work done over the last 30 years on the evolution of the upper Missouri River drainage in southwest Montana. The Missouri as we know it today follows a very recent drainage pattern, that began developing around 17 million years ago, coincident with the outbreak of the Yellowstone hot spot. The river followed extensional basins developed on the thermal bulge, flowing northeast into Canada. As the North American plate moved slowly southwest over the stationary hot spot, it impacted the types of sediments deposited in the basins, recorded as Miocene fluvial deposits of the Sixmile Creek Formation. By 4.5 million years ago, the relative position of the hot spot moved past southwest Montana and the thermal bulge collapsed, forming NW-trending basin and range topography. The old NE-trending river system was diverted in places into the new NW-trending grabens, creating the present complex composite drainage. The Sixmile Creek Formation is exposed in many of the mountain ranges raised in the path of the river, showing us the mountains are much younger than previously considered.