For some twenty years before the advent of Lewis and Clark, French-Canadian voyageurs of the North West Company were in league with the Mandans, and from these Indians learned of the distant "Pierre Jaune" or "Roche Jaune" River, a translation from the Indian equivalent of "Yellow Rock River." Chittenden theorizes that the ultimate origin of the name descends from the brilliant and infinite varieties of yellow which dominate the color scheme of the Grand Canyon of the Yellowstone, and which probably awed the first aboriginal explorer just as it does today's auto-borne tourist.

Although there is room for debate as to whether any of the Canadian traders beat Lewis and Clark to the mouth of the Yellowstone, it is certain that one of their number preceded the Americans in the approach to its headwaters. On September 10, 1805, Francois Antoine Larocque reached "Riviere aux Roches Jaunes" just below the mouth of Pryor's Fork, near present Billings, Montana, in the course of "a voyage of discovery to the Rocky Mountains." After wintering at the Mandan villages in 1804-1805 as a neighbor of the hibernating Lewis and Clark, and being thwarted in his desire to accompany them upstream, Larocque had returned to his post on the Assiniboine for supplies, then hurried back to the Mandans, going from there overland via Knife River, the Little Missouri, and the Tongue to the Bighorn Mountains, country of the Crows.

While wintering with the Mandans, Captain Clark sketched two maps of the unexplored country westward, based on "the information of traders, indians and my own observation and ideas." One of these shows "Rochejhone River" with six tributaries from the south, five with Indian names, two translated as "Tongue River" and "Big Horn R." The Bighorns and Rocky Mountains beyond are represented only by diagrammatic strokes. There is a trail from the mouth of Knife River to the Bighorns, roughly the same subsequently taken by Larocque. This was actually a refinement of a sketch made for Clark by the Mandan Chief Big White. The second map shows "River yellow rock" minus tributaries but with t