An early morning aerial view southward.
The Missouri River flows from right center toward lower left.
The Yellowstone enters at left of center, flowing north.
I ascended the hills from whence I had a most pleasing view of the country, perticularly of the wide and fertile vallies formed by the missouri and the yellowstone rivers, which occasionally unmasked by the wood on their borders disclose their meanderings for many miles in their passage through these delightfull tracts of country. . . . The whol face of the country was covered with herds of Buffaloe, Elk & Antelopes; the deer are also abundant, but keep themselves more concealed in the woodland. The buffalo Elk and Antelope are so gentle that we pass near them while feeding, without apearing to excite any alarm among them, and when we attract their attention, they frequently approach us more nearly to discover what we are, and in some instances pursue us a considerable distance apparently with that [in] view.
After leaving Fort Mandan the Corps of Discovery had passed through a "game sink" where animals were comparatively scarce, and now were entering a huge buffer zone between competing Indian tribes, a zone that historian Dan Flores has called "the American Serengeti." From the middle of April, when they passed the mouth of the Little Knife River (they called it "Goat Pen Creek") until the first week in June at the mouth of the Marias River, they were pacing off the width of a region overflowing with wild mega-fauna that would dwindle to near extinction before the end of the 19th century.
On April 26, 1805, Lewis wrote, "The Indians inform [us] that the Yellowstone River is navigable for perogues [large rowboats] nearly to its source in the Rocky Mountains, and that in its course near these mountains it passes within less than half a day's march of a navigable part of the Missouri."
Fifteen months later it would take Captain Clark only a little over two days to march from the Three Forks of the Missouri across the Gallatin Valley to the Yellowstone River at today's Livingston, Montana. It's 57 miles via modern Interstate 90, which parallels most of the Indian roads he followed. As to the navigability of the Yellowstone, it depends upon what the Hidatsas understood by "navigable," since they generally used boats—small "bull boats" consisting of willow frames covered with bison hide—mainly to cross rivers, not to go up or down.
Paul S. Martin and Christine R. Szuter, "War Zones and Game Sinks in Lewis and Clark's West," Conservation Biology, Vol. 13 No. 1 (February 1999), 36-45.