Geology of the Missouri River

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 of 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 approximately 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 prior to 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.

Nicknamed the “Big Muddy”, the Missouri certainly lives up to this name, carrying 20,000,000 to 25,000,000 short tons (18,000,000 to 23,000,000 t) of sediment per year. Before the construction of dams and levees, this load was 13-16 times higher, averaging 320,000,000 short tons (290,000,000 t) per year.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, channeling and diking the river has kept it from reaching its natural sediment sources along most of its course. Also, the creation of giant reservoirs has trapped millions of tons of sediment since the 1950s. 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.

Watershed of the Missouri River

With a drainage basin spanning 529,350 square miles (1,371,000 km²), the Missouri's catchment encompasses nearly one-sixth of the area of the United States or just over 5% of the continent of North America. The mostly flat, arid basin, comparable to the size of the Canadian province of Quebec, encompasses most of the northern Great Plains, stretching over 1,500 miles (2,400 km) from the Rocky Mountains in the west to the Mississippi River valley in the east. From north to south it reaches well over 1,600 miles (2,600 km), from the U.S.-Canada border in the north to the Arkansas River valley of the south. Compared with the Mississippi River above their confluence, the Missouri is actually much longer and drains a greater area. However, the flow of the Missouri at the confluence accounts for only 45% of the total amount of water below the meeting of the rivers.

Although vast, the Missouri River watershed is home to only about 10 million people, living in all of the U.S. state of Nebraska, parts of the U.S. states of Colorado, Iowa, Kansas, Minnesota, Missouri, Montana, North Dakota, South Dakota, and Wyoming, and small southern portions of the Canadian provinces of Alberta and Saskatchewan. Significant tributary systems within the basin include the Milk and Yellowstone in the northwest, the James and Osage of the east, and the Platte and Kansas-Republican/Smoky Hill drainages of the southwestern plains. The Platte is the longest tributary, but the Yellowstone River is the largest tributary by discharge (in fact the Yellowstone's flow is about 13,800 cubic feet per second (390 m³/s) while the Platte averages a comparatively mere 7,000 cubic feet per second (200 m³/s)).

Elevations in the watershed vary widely from just over 400 feet (120 m) at the Missouri's mouth to 12,000 to 14,500 feet (3,700 to 4,400 m) in the heights of the Rockies in Montana, Wyoming and Colorado. The river itself drops from over 9,000 feet (2,700 m) in elevation at Brower's Spring, the farthest source. Although the plains of the watershed appear flat to the eye, there is a definite slope from west to east; the land is roughly 3,000 feet (910 m) above sea level at the base of the Rockies, but less than 500 feet (150 m) at the border of the Mississippi valley. Mount Elbert, in Colorado, is the highest peak in the Missouri watershed at 14,433 feet (4,399 m) and is the 2nd highest in the Continental United States.

As one of the continent's most important rivers, the Missouri's drainage basin borders on many other major watersheds of the United States and Canada. The Columbia River and Colorado River systems drain the area west of the Rocky Mountains; however, in Wyoming, between the Missouri and Colorado watersheds there is a 3,900-square-mile (10,000 km²) endorheic drainage called the Great Divide Basin, that does not have an outlet to the sea. On the north it is bounded by the Saskatchewan and Red River of the North, as well as several large endorheic areas in southern Alberta, Saskatchewan and northeastern North Dakota. On the east it is bordered by the upper Mississippi River and its western tributaries, and to the south by the Arkansas and White watersheds.

Course of the Missouri River

The Missouri River rises in the Rocky Mountains of southwestern Montana at the confluence of the Jefferson River and Madison River, in Missouri Headwaters State Park near the town of Three Forks. It flows north, passing through Canyon Ferry Lake, west of the Big Belt Mountains. It exits from the mountains near Cascade, and flows northeast to the city of Great Falls, where it drops over the Great Falls of the Missouri, a series of five substantial cataracts. The Missouri then flows east through canyons and badlands, past the confluence with the Marias River, widening into the Fort Peck Lake reservoir near the confluence of the Musselshell River. Farther on the river passes through the gates of the Fort Peck Dam, and immediately after that, the Milk River enters from the left.

It flows eastwards through the plains of eastern Montana, receiving the Poplar River at Poplar before crossing into North Dakota where the Yellowstone River, its greatest tributary, joins from the right. At the confluence, the two rivers are approximately the same size. The Missouri then meanders east past Williston and into Lake Sakakawea, a reservoir formed by the Garrison Dam, where the river turns south. South of Riverdale, the Knife River enters from the right. The Missouri continues south to the city of Bismarck, the North Dakota state capital, where it receives the Heart River. It slows into the Lake Oahe reservoir just before the Cannonball River confluence. While it continues south, eventually reaching Oahe Dam in South Dakota, the Grand, Moreau and Cheyenne Rivers all join the Missouri from the right.

The Missouri makes a bend to the southeast as it meanders through the Great Plains, receiving the Niobrara River from the right. The Missouri then proceeds to form the boundary of South Dakota and Nebraska, then after it receives the James River, forms the Iowa-Nebraska boundary. At Sioux City the Big Sioux River comes in from the left. The Missouri flows south to the big city of Omaha where it receives one of its largest tributaries, the Platte River. Downstream of there, it begins to define the Nebraska-Missouri border, then flows between Missouri and Kansas. It turns east at Kansas City, where the Kansas River enters from the right, and so on into north-central Missouri. The river passes south of Columbia and receives the Osage River, its last major tributary, downstream of Jefferson City. It then rounds the northern side of St. Louis to join the Mississippi River on the border between Missouri and Illinois.

Missouri River

The Missouri River is a major river of central North America, and is a tributary of the Mississippi River. It is the longest river by name on the continent at 2,341 miles (3,767 km), and the second largest tributary of the Mississippi by discharge, after the Ohio. The watershed of the Missouri River drains nearly 530,000 square miles (1,400,000 km²) of the Great Plains east of the Rocky Mountains, spanning parts of ten U.S. states and two Canadian provinces. Around 10 million people live in the basin, many concentrated in urban centers along the main stem such as St. Louis; Kansas City; Omaha; Sioux City; and Great Falls. Measured from its hydrologic source in the Centennial Mountains of Montana to the Mississippi's mouth at the Gulf of Mexico, it forms part of the fourth-longest river system in the world.

As early as 12,000 years ago, Paleo-Indians settled in the plains of the Missouri River basin. Prominent Native American tribes that lived on the river in immediate pre-Columbian times included the Mandan, Sioux, Hidatsa, Osage, and Missouria – the latter for whom the river is named. European and American explorers wandered the region in the 1700s and 1800s, when the Missouri basin became part of France's Louisiana territory. When Louisiana was sold to the United States, the Lewis and Clark Expedition traveled the river in search of a route to the Pacific coast of North America. Settlers' expansion into the Great Plains pushed Native Americans out of their traditional lands, leading to wars. The Missouri River served as a boundary for the American frontier in the 19th century, and many prominent westward routes such as the Oregon Trail had their starting points on the river.

Although it once was by far the longest river of North America, today its length is comparable with the Mississippi because of channelization to eliminate meanders and facilitate navigation. The lower Missouri valley has become a highly productive agricultural and industrial region. Barges shipping gravel, wheat, fertilizer, and other grown, mined or manufactured products provide most river commerce today. In response to the growing amount of water traffic, federal and state agencies including the U.S. Army Corps of Engineers (USACE) heavily dammed and channelized the river in the 20th century. Although this development has contributed to the region's economic growth, it has taken a toll on the ecology and the water quality of the Missouri.