Captain Meriwether Lewis and Lieutenant (de facto Co-Captain and posthumously, officially promoted to Captain in advance of the bicentennial) William Clark commanded the Corps of Discovery to map the course of the Missouri River to its source and the Pacific Northwest overland and water routes to and from the mouth of the Columbia River. They were honored with a 3-cent stamp July 24, 1954 on the 150th anniversary. The 1803 Louisiana Purchase doubled the size of the United States. Lewis and Clark described and sketched its flora and fauna and described the native inhabitants they encountered before returning to St. Louis in 1806.