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.

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