The Missouri River runs through the states of Missouri, North and South Dakota, Nebraska, Iowa, Montana and Kansas. It's typically considered the second longest river in the United States, although some geologists believe it is the longest river, whereas the Mississippi has the largest drainage area. The Missouri River is about 2,315 miles long, with a drainage area that's over 1/2 million square miles.
The river's drainage area is inhabited by 10 million people. They include the people who live in the states the river flows through, but also those in Colorado, Minnesota, Wyoming and Alberta and Saskatchewan, two Canadian provinces.
The Missouri River has been important to human beings for at least 12,000 years and has been so developed in the modern era that its course has been changed several times. This has largely been through the building of dams, channels and levees. The river has also been polluted due to agricultural run-off and other human activities.
The headwaters of the Missouri River begin in three streams that flow from the Madison, Gallatin and Jefferson Rivers in Montana. From there the river widens, and then flows southeast to join the Mississippi River at St. Louis.
The Missouri gets its name from the Missouri Indians, whose name means "people with wooden canoes."