Missouri (river, United States)
Missouri, river, c.2,565 mi (4,130 km) long (including its Jefferson-Beaverhead-Red Rock headstream), the longest river of the United States and the principal tributary of the Mississippi River. The length of the combined Missouri-Mississippi system from the headwaters of the Missouri to the mouth of the Mississippi is c.3,740 mi (6,020 km), making it the world's third longest river after the Nile and the Amazon. The Missouri River drains an area of c.580,000 sq mi (1,502,200 sq km), including 2,550 sq mi (6,600 sq km) in Canada.
The principal headwaters of the Missouri are the Jefferson, Madison, and Gallatin rivers, which rise high in the Rocky Mts., SW Mont., and join to form the Missouri near Three Forks, Mont. The Missouri's upper course flows north through scenic mountain terrain including Gate of the Mountains, a deep gorge. At Great Falls, Mont., the river enters a 10-mi (16-km) stretch of cataracts that prevented navigation to the upper river and effectively established Fort Benton, Mont., as the head of navigation for 19th-century riverboats. Below Fort Benton the Missouri follows a meandering course east through the unspoiled Missouri Breaks and Fort Peck Lake (behind Fort Peck Dam) then southeast through the dammed Lakes Sakakawea and Oahe and across the Great Plains of the W central United States, crossing Montana, North Dakota, and South Dakota, and forming part of the boundaries of Nebraska, Kansas, and Iowa before crossing Missouri and entering the Mississippi River 17 mi (27 km) N of St. Louis. Nicknamed "Big Muddy" for its heavy load of silt, the brown waters of the Missouri do not readily mix with the gray waters of the Mississippi until c.100 mi (160 km) downstream. The Yellowstone and Platte rivers are the Missouri's chief tributaries.
Human Impact and Use
Above Sioux City, Iowa, the Missouri's fluctuating flow is regulated by seven major dams (Gavins Point, Fort Randall, Big Bend, Oahe, Garrison, Fort Peck, and Canyon Ferry) and more than 80 other dams on tributary streams. These dams, with their reservoirs, are part of the coordinated, basin-wide Missouri River basin project (authorized by the U.S. Congress in 1944), which provides for flood control, hydroelectric power, irrigation water, and recreational facilities. The dams serve to impound for later use the spring rains and snowmelt that swell the volume of the river in March and April and also the second flood stage that frequently occurs in June as the snow melts in the remoter mountain regions. Despite this system of dams, during the extremely rainy summer of 1993 the lower Missouri reached record levels, flooding many areas, eroding farmland, and depositing huge quantities of sand that damaged many thousands of acres of fertile bottomland. Flooding was also a significant problem along the river in 2011.
Since the dams have no locks, Sioux City is the head of navigation for the 9-ft (2.7-m) channel maintained over the 760-mi (1,223-km) stretch downstream to the Mississippi. Tugboats pushing strings of barges move freight along this route. From December to March, navigation is interrupted by ice and low water levels (resulting from upstream freezing); summer water levels, which frequently fell so low as to cause river boats to go aground, are now maintained at safe levels by the release of water from Gavin Point Dam. Silt, fertilizers, and pesticides, which are contained in the runoff from agricultural lands, pollute the water above Sioux City, but wastes from industrial plants and from inadequately treated municipal sewage create a more serious level of pollution downstream. There has been a reduction in wetland areas and a loss of fish and wildlife due to the damming of the river.
The Missouri River was an important artery of commerce for Native American villages of the Plains culture long before Europeans arrived. The French explorers Jacques Marquette and Louis Jolliet passed the mouth of the river in 1683 and the Canadian explorer Vérendrye visited the upper reaches of the river in 1738. David Thompson, a Canadian fur trader, explored part of the river in 1797. Meriwether Lewis and William Clark followed the Missouri on their journey (1803–6) to the Pacific Ocean and described it at length (see Lewis and Clark expedition). The first steamboat ascended the river in 1819, and hundreds more later navigated the uncertain waters to Fort Benton. Mormons bound for Utah and pioneers bound for Oregon and California followed the Missouri valley and that of the Platte overland to the West. River traffic declined with the loss of freight to the railroads after the Civil War. Although it was revitalized in the mid-20th cent., in the section below Sioux City, through the navigational improvements and flood control efforts of the Missouri River basin project, barge traffic declined in the late 20th cent. Two stretches of the river are protected as the Missouri National Recreational River (see National Parks and Monuments (table)).
See B. De Voto, Across the Wide Missouri (1947, repr. 1972); H. M. Chittenden, Early Steamboat Navigation on the Missouri River (1972); B. Priddy, Across Our Wide Missouri (2 vol., 1982–84).