The Missouri River is the longest river in North America. Rising in the Rocky Mountains of western Montana, the Missouri flows east and south for 〔 before entering the Mississippi River north of St. Louis, Missouri. The river takes drainage from a sparsely populated, semi-arid watershed of more than half a million square miles (1,300,000 km2), which includes parts of ten U.S. states and two Canadian provinces. When combined with the lower Mississippi River, it forms the world's fourth longest river system.〔
For over 12,000 years, people have depended on the Missouri and its tributaries as a source of sustenance and transportation. More than ten major groups of Native Americans populated the watershed, most leading a nomadic lifestyle and dependent on enormous buffalo herds that once roamed through the Great Plains. The first Europeans encountered the river in the late seventeenth century, and the region passed through Spanish and French hands before finally becoming part of the United States through the Louisiana Purchase. The Missouri was long believed to be part of the Northwest Passage – a water route from the Atlantic to the Pacific – but when Lewis and Clark became the first to travel the river's entire length, they confirmed the mythical pathway to be no more than a legend.
The Missouri was one of the main routes for the westward expansion of the United States during the 19th century. The growth of the fur trade in the early 1800s laid much of the groundwork as trappers explored the region and blazed trails. Pioneers headed west ''en masse'' beginning in the 1830s, first by covered wagon, then by the growing numbers of steamboats entering service on the river. Former Native American lands in the watershed were taken over by settlers, leading to some of the most longstanding and violent wars against indigenous peoples in American history.
During the 20th century, the Missouri River basin was extensively developed for irrigation, flood control and the generation of hydroelectric power. Fifteen dams impound the main stem of the river, with hundreds more on tributaries. Meanders have been cut and the river channelized to improve navigation, reducing its length by almost from pre-development times. Although the lower Missouri valley is now a populous and highly productive agricultural and industrial region, heavy development has taken its toll on wildlife and fish populations as well as water quality.
From the Rocky Mountains of Montana and Wyoming, three streams rise to form the headwaters of the Missouri River. The longest begins near Brower's Spring, above sea level, on the southeastern slopes of Mount Jefferson in the Centennial Mountains. Flowing west then north, it runs first in Hell Roaring Creek, then west into the Red Rock; swings northeast to become the Beaverhead, it finally joins with the Big Hole to form the Jefferson. The Firehole River originates at Madison Lake in Wyoming's Yellowstone National Park and joins with the Gibbon to form the Madison, while the Gallatin River rises out of Gallatin Lake, also in the national park. These two streams then flow north and northwest into Montana.〔〔
The Missouri River officially starts at the confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks, Montana, and is joined by the Gallatin a mile (1.6 km) downstream. The Missouri then passes through Canyon Ferry Lake, a reservoir west of the Big Belt Mountains. Issuing from the mountains near Cascade, the river flows northeast to the city of Great Falls, where it drops over the Great Falls of the Missouri, a series of five substantial waterfalls. It then winds east through a scenic region of canyons and badlands known as the Missouri Breaks, receiving the Marias River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River. Farther on, the river passes through the Fort Peck Dam, and immediately downstream, the Milk River joins from the north.
Flowing eastwards through the plains of eastern Montana, the Missouri receives the Poplar River from the north before crossing into North Dakota where the Yellowstone River, its greatest tributary by volume, joins from the southwest. At the confluence, the Yellowstone is actually the larger river. The Missouri then meanders east past Williston and into Lake Sakakawea, the reservoir formed by Garrison Dam. Below the dam the Missouri receives the Knife River from the west and flows south to Bismarck, the capital of North Dakota, where the Heart River joins from the west. 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 west.〔〔
The Missouri makes a bend to the southeast as it winds through the Great Plains, receiving the Niobrara River and many smaller tributaries from the southwest. It then proceeds to form the boundary of South Dakota and Nebraska, then after being joined by the James River from the north, forms the Iowa–Nebraska boundary. At Sioux City the Big Sioux River comes in from the north. The Missouri flows south to the city of Omaha where it receives its longest tributary, the Platte River, from the west. Downstream, it begins to define the Nebraska–Missouri border, then flows between Missouri and Kansas. The Missouri swings east at Kansas City, where the Kansas River enters from the west, and so on into north-central Missouri. To the east of the Kansas City, Missouri receives, on the left side, the Grand River. It passes south of Columbia and receives the Osage and Gasconade Rivers from the south downstream of Jefferson City. The river then rounds the northern side of St. Louis to join the Mississippi River on the border between Missouri and Illinois.〔〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』