Missouri River - Watershed

Watershed

There is only one river with a personality, a sense of humor, and a woman's caprice; a river that goes traveling sidewise, that interferes in politics, rearranges geography, and dabbles in real estate; a river that plays hide and seek with you today and tomorrow follows you around like a pet dog with a dynamite cracker tied to his tail. That river is the Missouri.
-George Fitch, circa 1840

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 five percent of the continent of North America. Comparable to the size of the Canadian province of Quebec, the watershed encompasses most of the central Great Plains, stretching from the Rocky Mountains in the west to the Mississippi River Valley in the east and from the southern extreme of western Canada to the border of the Arkansas River watershed. Compared with the Mississippi River above their confluence, the Missouri is twice as long and drains an area three times as large. The Missouri accounts for 45 percent of the annual flow of the Mississippi past St. Louis, and as much as 70 percent in certain droughts.

As of 1990, the Missouri River watershed was home to about 12 million people. This includes the entire population 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. The watershed's largest city is Denver, Colorado, with a population of more than six hundred thousand. Denver is the main city of the Front Range Urban Corridor whose cities had a combined population of over four million in 2005, making it the largest metropolitan area in the Missouri River basin. Other major population centers – mostly located in the southeastern portion of the watershed – include Omaha, Nebraska, situated north of the confluence of the Missouri and Platte Rivers; Kansas City, Missouri – Kansas City, Kansas, located at the confluence of the Missouri with the Kansas River, and the St. Louis metropolitan area, situated south of the Missouri River just above its mouth on the Mississippi. In contrast, the northwestern part of the watershed is sparsely populated. However, many northwestern cities, such as Billings, Montana, are among the fastest growing in the Missouri basin.

With more than 170,000 square miles (440,000 km2) under the plow, the Missouri River watershed includes roughly one-fourth of all the agricultural land in the United States, providing more than a third of the country's wheat, flax, barley and oats. However, only 11,000 square miles (28,000 km2) of farmland in the basin is irrigated. A further 281,000 square miles (730,000 km2) of the basin is devoted to the raising of livestock, mainly cattle. Forested areas of the watershed, mostly second-growth, total about 43,700 square miles (113,000 km2). Urban areas, on the other hand, comprise less than 13,000 square miles (34,000 km2) of land. Most built-up areas are located along the main stem and a few major tributaries, including the Platte and Yellowstone Rivers.

Elevations in the watershed vary widely, ranging from just over 400 feet (120 m) at the Missouri's mouth to the 14,278-foot (4,352 m) summit of Grays Peak in central Colorado. The river itself drops a total of 8,626 feet (2,629 m) from Brower's Spring, the farthest source. Although the plains of the watershed have extremely little local vertical relief, the land rises about 10 feet per mile (1.9 m/km) from east to west. The elevation is less than 500 feet (150 m) at the eastern border of the watershed, but is over 3,000 feet (910 m) above sea level in many places at the base of the Rockies.

The Missouri's drainage basin has highly variable weather and rainfall patterns, Overall, the watershed is defined by a Continental climate with warm, wet summers and harsh, cold winters. Most of the watershed receives an average of 8 to 10 inches (200 to 250 mm) of precipitation each year. However, the westernmost portions of the basin in the Rockies as well as southeastern regions in Missouri may receive as much as 40 inches (1,000 mm). The vast majority of precipitation occurs in winter, although the upper basin is known for short-lived but intense summer thunderstorms such as the one which produced the 1972 Black Hills flood through Rapid City, South Dakota. Winter temperatures in Montana, Wyoming and Colorado may drop as low as −60 °F (−51 °C), while summer highs in Kansas and Missouri have reached 120 °F (49 °C) at times.

As one of the continent's most significant river systems, the Missouri's drainage basin borders on many other major watersheds of the United States and Canada. The Continental Divide, running along the spine of the Rocky Mountains, forms most of the western border of the Missouri watershed. The Clark Fork and Snake River, both part of the Columbia River basin, drain the area west of the Rockies in Montana, Idaho and western Wyoming. The Columbia, Missouri and Colorado River watersheds meet at Three Waters Mountain in Wyoming's Wind River Range. South of there, the Missouri basin is bordered on the west by the drainage of the Green River, a tributary of the Colorado, then on the south by the mainstem of the Colorado. Both the Colorado and Columbia Rivers flow to the Pacific Ocean. However, a large endorheic drainage called the Great Divide Basin exists between the Missouri and Green watersheds in western Wyoming. This area is sometimes counted as part of the Missouri River watershed, even though its waters do not flow to either side of the Continental Divide.

To the north, the much lower Laurentian Divide separates the Missouri River watershed from those of the Oldman River, a tributary of the South Saskatchewan River, as well as the Souris, Sheyenne, and smaller tributaries of the Red River of the North. All of these streams are part of Canada's Nelson River drainage basin, which empties into Hudson Bay. There are also several large endorheic basins between the Missouri and Nelson watersheds in southern Alberta and Saskatchewan. The Minnesota and Des Moines Rivers, tributaries of the upper Mississippi, drain most of the area bordering the eastern side of the Missouri River basin. Finally, on the south, the Ozark Mountains and other low divides through central Missouri, Kansas and Colorado separate the Missouri watershed from those of the White River and Arkansas River, also tributaries of the Mississippi River.