Missouri River, one of the largest rivers in North America and a chief tributary of the Mississippi. It is also called the “Big Muddy,” because of the tremendous load of silt that it carries. The Missouri begins at the junction of the Jefferson, Madison, and Gallatin rivers near Three Forks, Montana. It flows 2,315 miles (3,726 km) to the Mississippi, the junction being just north of St. Louis, Missouri.

Including the Jefferson-Beaverhead-Red Rock headwater stream, which has its source in the Rocky Mountains west of Yellowstone National Park, the Missouri's length is 2,564 miles (4,126 km). The combined Mississippi-Missouri-Red Rock, with a length of 3,741 miles (6,021 km), is one of the world's great rivers.