Geology
The Rocky Mountains of southwestern Montana at the headwaters of the Missouri River first rose in the Laramide Orogeny, a mountain-building episode that occurred from around 70 to 45 million years ago (the end of the Mesozoic through the early Cenozoic). This orogeny uplifted Cretaceous rocks along the western side of the Western Interior Seaway, a vast shallow sea that stretched from the Arctic Ocean to the Gulf of Mexico, and deposited the sediments that now underlie much of the drainage basin of the Missouri River. This Laramide uplift 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 Orogeny 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.
The Missouri and many of its tributaries cross the Great Plains, flowing over or cutting into the Ogallala Group and older mid-Cenozoic sedimentary rocks. The lowest major Cenozoic unit, the White River Formation, was deposited between roughly 35 and 29 million years ago and consists of claystone, sandstone, limestone, and conglomerate. Channel sandstones and finer-grained overbank deposits of the fluvial Arikaree Group were deposited between 29 and 19 million years ago. The Miocene-age Ogallala and the slightly younger Pliocene-age Broadwater Formation deposited atop the Arikaree Group, and are formed from material eroded off of the Rocky Mountains during a time of increased generation of topographic relief; these formations stretch from the Rocky Mountains nearly to the Iowa border and give the Great Plains much of their gentle but persistent eastward tilt, and also constitute a major aquifer.
Immediately before the Quaternary Ice Age, the Missouri River was likely split into three segments: an upper portion that drained northwards into Hudson Bay, and middle and lower sections that flowed eastward down the regional slope. As the Earth plunged into the Ice Age, a pre-Illinoian (or possibly the Illinoian) glaciation diverted the Missouri River southeastwards towards its present confluence with the Mississippi and caused it to integrate into a single river system that cuts across the regional slope. In western Montana, the Missouri River is thought to have once flowed north then east around the Bear Paw Mountains. Sapphires are found in some spots along the river in western Montana. Advances of the continental ice sheets diverted the river and its tributaries, causing them to pool up into large temporary lakes such as Glacial Lakes Great Falls, Musselshell and others. As the lakes rose the water in them often spilled across adjacent local drainage divides, creating now-abandoned channels and coulees including the Shonkin Sag, 100 miles (160 km) long. When the glaciers retreated, the Missouri flowed in a new course along the south side of the Bearpaws and the lower part of the Milk River tributary took over the original main channel.
The Missouri's nickname, the "Big Muddy", was inspired by its enormous loads of sediment or silt – some of the largest of any North American river. In its pre-development state, the river transported some 175–320 million short tons (193–290 million t) per year. The construction of dams and levees has drastically reduced this to 20–25 million short tons (18–23 million t) in the present day. Much of this sediment is derived from the river’s floodplain, also called the meander belt; every time the river changed course, it would erode tons of soil and rocks from its banks. However, damming and channeling the river has kept it from reaching its natural sediment sources along most of its course. Reservoirs along the Missouri trap roughly 36.4 million short tons (32.9 million t) of sediment each year. Despite this, the river still transports more than half the total silt that empties into the Gulf of Mexico; the Mississippi River Delta, formed by sediment deposits at the mouth of the Mississippi, constitutes a majority of sediments carried by the Missouri.
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