In his summary of the Yellowstone's attractions on July 30, 1806, Clark directed most of his attention toward opportunities for immediate expansion of the fur trade. "Like all other branches of the missouri which penetrate the Rocky Mountains, all that portion of it lying within those mountains abound in fine beaver and Otter." The main advantage of the Yellowstone as a trade route was that it lay well south of the Missouri River and the homelands of the Blackfeet and their allies, the Atsinas, who together constituted the greatest threat to commercial enterprise, at least in Clark's view.
"To an establishment on this river at Clarks Fork [of the Yellowstone], the Shoshones both within and West of the Rocky Mountains would willingly resort for the purposes of trade," Clark concluded.
The only other reasonable alternative was to build on the eastern fringe of the hostile territory, opposite the mouth of the Yellowstone. In fact, even before the captains saw it, they were convinced of that. Lewis wrote, at Fort Mandan:
Fort Union would be established on the eastern fringe of that region, opposite the mouth of the Yellowstone, in 1829,2 and another 21 years would pass before the trading post called Fort Benton would be founded, in 1850, at the head of navigation on the Missouri.
Meanwhile, the rest of the Yellowstone River beckoned. Even as Clark descended it, Manuel Lisa, back in St. Louis, was thinking of making an ascent, and in 1807 would build the first trading post on the Yellowstone--at the mouth of the Bighorn, not the Clark's Fork.3
Lewis and Clark got more from the Mandans and Hidatsas than basic directions and lists of landmarks. Whether through certain questions the captains asked, or just from the tone of voice with which the Indians poke of the land they knew, or else out of that sincere congeniality that prompts people to share their feelings with welcome guests, the explorers gained a sense of the quality and beauty of the Yellowstone Valley. "If Indian information can be relied on," wrote Lewis, "this river waters one of the fairest portions of Louisiana."4
1. Panopolize. Lewis probably meant monopolize – literally, "one seller" – but he evidently had a feel for Latin, for his word – "all seller" – works well enough, even though it has never become a "real" word.
2. Erwin N. Thompson, Fort Union Trading Post: Fur Trade Empire on the Upper Missouri (Williston, ND: Fort Union Association, 1994).
3. Richard Oglesby, Manuel Lisa and the Opening of the Missouri Fur Trade (Norman: University of Oklahoma Press, 1963).
4. Moulton, ed., Journals, 3:363–65.