In December 1803, William Clark established "Camp River Dubois" at the confluence of the Mississippi and Missouri Rivers, north of St. Louis. While there he recruited and trained men, while Meriwether Lewis spent time in St. Louis, conferring with traders about the Upper Missouri regions and obtaining maps made by earlier explorers.
On May 14, 1804, William Clark and the Corps of Discovery left Camp River Dubois, and were joined by Meriwether Lewis in St. Charles, Missouri. The party numbered over 45, and included 27 young, unmarried soldiers, a French-Indian interpreter, and Clark's Black slave York. An additional group of men, engagés, would travel only to the Mandan country for the first winter, and these included six soldiers and several French boatmen.