 In 1682 , France expanded its territorial claims in North America to include land on the western side of the Mississippi River , which included the lower portion of the Missouri . However , the Missouri itself remained formally <unk> until Étienne de <unk> , Sieur de Bourgmont commanded an expedition in 1714 that reached at least as far as the mouth of the Platte River . It is unclear exactly how far Bourgmont traveled beyond there ; he described the blond @-@ haired <unk> in his journals , so it is likely he reached as far as their villages in present @-@ day North Dakota . Later that year , Bourgmont published The Route To Be Taken To <unk> The Missouri River , the first known document to use the name " Missouri River " ; many of the names he gave to tributaries , mostly for the native tribes that lived along them , are still in use today . The expedition 's discoveries eventually found their way to <unk> Guillaume <unk> , who used the information to create a map of the lower Missouri . In <unk> , Jean @-@ Baptiste Le <unk> , Sieur de <unk> requested that the French government bestow upon Bourgmont the Cross of St. Louis because of his " outstanding service to France " . 
