MISSOURI RIVER

The Missouri River is the longest river in the United States. It flows for 2,466 miles (3,968 kilometers). Its source lies in the Rocky Mountains of southwestern Montana; the river is formed by the confluence of the Jefferson, Madison, and Gallatin rivers at Three Forks, Montana. From there the Missouri flows east and southeast, ultimately joining the Mississippi River about 10 miles (16 kilometers) north of St. Louis, Missouri. The Missouri River flows through Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, and Missouri. From the Mississippi River, the Missouri is navigable by barges and towboats west and north as far as Sioux City, Iowa. Above Sioux City the water flow is controlled by a series of dams in a project authorized by the U.S. government in 1944. When the water is high the river is navigable to Great Falls (northeast of Helena), Montana.

In 1673 the mouth of the Missouri was passed by French-Canadian adventurer Louis Jolliet (1645–1700) and French missionary Jacques Marquette (1637–75) as they explored the upper part of the Mississippi River. The Lewis and Clark expedition of 1804–06 followed the Missouri for much of the journey to the Pacific Ocean.

During the first two decades of the 1800s, the river provided a chief transportation route for the western fur trade, which relied on keel boats to move goods along the river. In 1819 steamboat traffic began on the waterway, carrying pioneers to the rugged West. Riverboat traffic declined with the expansion of the cross-continental railroads at the end of the nineteenth century. Much of the region through which the Missouri River flows—the interior plains—was the last frontier to be settled in America.