 <unk> the most successful of the Missouri Company expeditions was that of James MacKay and John Evans . The two set out along the Missouri , and established Fort Charles about 20 miles ( 32 km ) south of present @-@ day Sioux City as a winter camp in 1795 . At the Mandan villages in North Dakota , they expelled several British traders , and while talking to the populace they <unk> the location of the Yellowstone River , which was called Roche <unk> ( " Yellow Rock " ) by the French . Although MacKay and Evans failed to accomplish their original goal of reaching the Pacific , they did create the first accurate map of the upper Missouri River . 
