A river of the United States rising in the Rocky Mountains as various headstreams that join to form the Missouri proper in southwest Montana. The longest river in the United States, it flows about 3,750 km (2,350 mi) in a meandering course to the Mississippi River north of St. Louis, Missouri. French explorers first reached the river in the late 1600s, and Lewis and Clark followed it on their journey westward (1804-1806).