 Fur trapping activities in the early 19th century encompassed nearly all of the Rocky Mountains on both the eastern and western slopes . <unk> of the Hudson 's Bay Company , St. Louis Missouri Fur Company , American Fur Company , Rocky Mountain Fur Company , North West Company and other outfits worked thousands of streams in the Missouri watershed as well as the neighboring Columbia , Colorado , Arkansas , and Saskatchewan river systems . During this period , the trappers , also called mountain men , blazed trails through the wilderness that would later form the paths pioneers and settlers would travel by into the West . Transport of the thousands of beaver pelts required ships , providing one of the first large motives for river transport on the Missouri to start . 
