What is the longest river in the United States?

The Missouri River is the longest river in the United States, flowing about 2,341 miles from its sources in the Rocky Mountains of southwestern Montana to its junction with the Mississippi River just north of St. Louis, slightly longer than the Mississippi itself at about 2,320 miles. The combined Missouri-Mississippi system from Three Forks, Montana to the Gulf of Mexico runs about 3,710 miles, the fourth longest river system in the world after the Nile, Amazon, and Yangtze.

The Missouri rises near Three Forks, Montana, where the Jefferson, Madison, and Gallatin rivers (named by the Lewis and Clark Expedition for President Thomas Jefferson, Secretary of State James Madison, and Treasury Secretary Albert Gallatin) merge to form the Missouri. The river flows north through Great Falls, Montana (where it drops over the Great Falls cataracts, formerly five large waterfalls now mostly dammed), east through Fort Peck Reservoir, southeast across North Dakota and South Dakota through Lake Sakakawea and Lake Oahe (the largest reservoirs on the river), south past Bismarck, Pierre, and Sioux City, then southeast across Nebraska forming much of the Iowa-Nebraska border at Omaha and Council Bluffs, then south to Kansas City and St. Louis where it joins the Mississippi.

The river drains about 529,000 square miles, more than 17 percent of the lower 48 states, including parts of Montana, Wyoming, North Dakota, South Dakota, Nebraska, Kansas, Iowa, Missouri, Colorado, and Minnesota. Major tributaries include the Yellowstone, Cheyenne, Platte, Kansas, and Osage rivers.

The Lewis and Clark Expedition followed the Missouri from St. Louis to its source between 1804 and 1805. The river was the principal westward route for fur trappers, settlers, and explorers in the early nineteenth century, and steamboats reached as far upstream as Fort Benton, Montana from the 1860s. The Missouri carries enormous sediment loads (giving it the nickname Big Muddy or Mighty Mo) and historically meandered widely across its floodplain, contributing to the rich soils of the Great Plains.

Major dams and reservoirs constructed by the Pick-Sloan Plan beginning in 1944 transformed the river. The Garrison, Oahe, Big Bend, Fort Randall, and Gavins Point dams in the Dakotas regulate flow, generate electricity, and provide flood control. Below Sioux City the river was channelized for navigation. Cities along the Missouri include Great Falls, Bismarck, Pierre, Sioux City, Omaha, Kansas City, and St. Louis (technically on the Mississippi just below the confluence). Devastating floods in 1993 and 2011 demonstrated the limits of flood control.

Indigenous peoples including the Mandan, Hidatsa, Arikara, Lakota, and Crow have lived along the Missouri for centuries.