 As early as the 18th century , fur trappers entered the extreme northern basin of the Missouri River in the hopes of finding populations of beaver and river otter , the sale of whose pelts drove the thriving North American fur trade . They came from many different places – some from the Canadian fur corporations at Hudson Bay , some from the Pacific Northwest ( see also : Maritime fur trade ) , and some from the <unk> United States . Most did not stay in the area for long , as they failed to find significant resources . 
