 Lisa 's men started construction of Fort Raymond , which sat on a bluff overlooking the confluence of the Yellowstone and Bighorn , in the fall of 1807 . The fort would serve primarily as a trading post for <unk> with the Native Americans for furs . This method was unlike that of the Pacific Northwest fur trade , which involved trappers hired by the various fur enterprises , namely Hudson 's Bay . Fort Raymond was later replaced by Fort Lisa at the confluence of the Missouri and Yellowstone in North Dakota ; a second fort also called Fort Lisa was built downstream on the Missouri River in Nebraska . In 1809 the St. Louis Missouri Fur Company was founded by Lisa in conjunction with William Clark and Pierre <unk> , among others . In 1828 , the American Fur Company founded Fort Union at the confluence of the Missouri and Yellowstone Rivers . Fort Union gradually became the main headquarters for the fur trade in the upper Missouri basin . 
