The Missouri River was the principal route of the Lewis and Clark Expedition through the Louisiana Purchase.
The expedition began in 1804 after President Thomas Jefferson commissioned Meriwether Lewis and William Clark to explore the newly acquired territory. The party traveled up the Missouri River, crossed the Rocky Mountains, and followed the Clearwater, Snake, and Columbia rivers to the Pacific Ocean.
The Missouri was essential because it formed the main east-to-west corridor into the interior from the Mississippi River. The explorers mapped waterways, recorded plants and animals, and established diplomatic contacts with many Native nations, although the expedition took place within lands already inhabited and governed by Indigenous peoples.
The expedition returned to St. Louis in 1806. It did not explore every part of the purchase, but its maps and reports strengthened American knowledge and later claims in the West. The Mississippi River was important to the expedition’s starting region, but the Missouri was its principal upstream route.