The journal Clark wrote as he descended the Yellowstone contained as much confirmation as discovery, for he and Lewis had been attentive and adept students of their Hidatsa and Mandan mentors during the winter of 1804-05, and they had left Fort Mandan on April 7, 1805, with an extraordinary amount of new, mainly generalized, if here and there misunderstood, knowledge about the land they were to explore.
From the Hidatsas they learned that the Meé,-ah'-zah—literally, "stone river"—or Yellowstone river, joined the E-mâh-tark'-Ah'-zhah, or "river creek" Missouri about 3 miles above the mouth of the Ok-hah-,Âh-zhâh (literally "white creek"), or White Earth river. They heard that the Yellowstone was as large as the Missouri, and more rapid, that it originated in the Rocky Mountains, near the source of "a river on which the Spaniards reside," though whether that was the Rio Grande or the Colorado was uncertain.
The Indians gave them a fairly clear picture of the three major reaches of the Yellowstone, with their distinctive characteristics. They learned that the river began in "broken ranges of the Rocky mountains....stoney, and thickly timbered, the vallies said to be wide in many places and the lands fertile." That was a reasonably accurate description of the stretch between Gardiner and Livingston that includes what is now known as Paradise Valley, which Clark did not explore.
They were told that from the "big bend" at Livingston, where the river emerges between Wineglass Mountain and Livingston Peak, and the Oke-tar-pas-ah-ha—a Mandan name of unknown meaning; later O'Fallon's Creek—the country was "more level, tho' still broken, fertile and well timbered." This was only partly true. As Clark was to find later, the soil was rocky and arid toward the lower end of this reach. He was back in the shortgrass prairie of the high plains.
Actually, the terraces of the valley floor are half gravel and stones, and half sand and silt. That's why some of their horses' hooves became so raw and bloody that the men had to fashion "Mockersons" of green (untanned) bison hide. As to timber, he saw only some scrubby pines on the mountainsides, and even in the river bottoms, he wrote, "I can See no timber Sufficient large for a Canoe which will Carry more than 3 men and such a one would be too Small to answer my purpose."
Near today's settlement of Park City, still within the reach he had understood the Indians to claim was "fertile and well timbered," he observed, "the plains are butifull and leavel but the Soil is but thin, Stoney, and in maney parts of the plains & bottoms there are great quantity of prickly pears." Indeed, the plains were so hard and dry that it was impossible to track the departing hoofprints when their horses were stolen.
Nonetheless, his optimism prevailed. When he reached the mouth of the Yellowstone on August 3, 1806, he insisted:
The Country through which it passes from those Mounts. to its junction is Generaly fertile rich open plains, the upper portion of which is roleing, and thehigh hills and hill sides are partially covered with pine and Stoney. The middle portion or from the enterance of Clarks Fork as low as the Buffalow shoals, the high lands Contain some Scattering pine on the Larboard Side. On the Starboard, or S. E. Side is Some hills thickly Supplied with pine.
One might say it was a mixed review.
John Logan Allen, Lewis and Clark and the Image of the American Northwest (New York: Dover, 1975), 242-44.