 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 midwestern United States . Most did not stay in the area for long , as they failed to find significant resources . 
 As early as the 18th century , fur trappers<a href='been_done' title='1'><mark></mark></a> 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 midwestern United States . Most did not stay in <a href='None' title='2'><mark>the</mark></a><a href='negative_concord' title='3'><mark>area</mark></a> for long , as they failed to find significant resources . 
As early as the 18th century , fur trappers done entered da extreme northern basin of tdaMissouri River in thdaopes of finding populations of beaver and river otter , thedale of whose pelts drove the thriving North American fur trade . They came from many diff places – some from the Canadian fur corporations at Hudson Bay , some from the Pacific Northwest ( see also : Maritime fur trade ) , and sum from the midwestern United States . Most did not stay in no area for long , as they failed to find significant resources .