Missouri River

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 4,127 km (2,565 mi) in a meandering course to the Mississippi River north of St. Louis, Missouri. French explorers first reached the river in the late 17th century, and Lewis and Clark followed it on their journey westward (1804-1806).