 For over 12 @,@ 000 years , people have depended on the Missouri River and its tributaries as a source of sustenance and transportation . More than ten major groups of Native Americans populated the watershed , most leading a nomadic lifestyle and dependent on enormous buffalo herds that once roamed through the Great Plains . The first Europeans encountered the river in the late seventeenth century , and the region passed through Spanish and French hands before finally becoming part of the United States through the Louisiana Purchase . The Missouri was long believed to be part of the Northwest Passage – a water route from the Atlantic to the Pacific – but when Lewis and Clark became the first to travel the river 's entire length , they confirmed the mythical pathway to be no more than a legend . 
