The Indians told them there were "no stream[s] worthy of notice which discharge themselves into this river on the N. side, the country between this river, and the Missouri being watered by the Mussle shell river." That was true as far as it went, but the Indians evidently forgot, or the captains lost in the translation, the river they would name after their prized blacksmith, gunsmith, and carpenter, Private John Shields, then 40 years old, and their senior enlisted man. The sources of the Shields River are shared by the Bridger Range and the Crazy Mountains (see Mountain Ranges in Montana and Idaho).
The captains wrote down the Indian names for at least a dozen tributaries of the Yellowstone, and Clark found most of them--the Powder, the Tongue, the Bighorn, Oke-tar-pas-ah-ha (later to become O'Fallon Creek, after William Clark's nephew, Benjamin O'Fallon), and (the Indians translated the name) Stinking Cabin Creek.
The best news at the time was that the Yellowstone was navigable as far as the Big Bend at Livingston. That would have been almost 500 miles above its confluence with the Missouri, and would have brought river commerce to within easy reach of the east fork of the Gallatin River, which also, by Indian standards, was "navigable" by small craft. Clark would estimate that portage to be only 18 miles long, "on an excellent high dry firm road with very inconsiderable hills, plus another 18 miles to the nearest part of the main fork of Gallitine... through a leavel plain."
His view of the Yellowstone as a navigable river grew more perfect in his mind as he descended it, and upon arrival back at the Missouri on August 3 he concluded: "The Rochejhone or Yellow Stone river is large and navagable with but fiew obstructions quite into the rocky mountains"--adding, despite Indian information to the contrary, plus the fact he had seen no evidence of this--"and probably near it's source."
This vision of the Yellowstone as a riverine superhighway would survive year by year until after mid-century. The commercial enterprises that were headed for the Yellowstone even as Clark was descending it, were traveling by the old reliable means--keelboats, bateaux, pirogues and canoes.