 Fur trapping activities in the early 19th century encompassed nearly all of the Rocky Mountains on both the eastern and western slopes . Trappers 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 . 
 Fur trapping activities in the early 19th century encompassed nearly all of the Rocky Mountains on both the eastern and western slopes . Trappers of the Hudson <a href='null_genetive' title='1'><mark>'s</mark></a>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 <a href='null_relcl' title='2'><mark>that</mark></a>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 . 
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