On the recommendation of the Choteau Fur Company, long the headquarters for mountaineers, Raynolds hired the "best guide," civilian Jim Bridger, to lead the expedition. Raynolds left Fort Pierre in June, traveling overland to Fort Sarpy on the Yellowstone River where they arrived in late August. The fur company had sent provisions upriver by bateaux to resupply Raynolds and deliver annuities for the Crow gathered at the fort. With Bridger as guide, the expedition departed Fort Sarpy and ascended the Yellowstone to the mouth of the Bighorn River where the expedition separated. Lt. Maynadier took a party and proceeded southeast to the Rosebud, Tongue, and Powder rivers and rejoined the main party on the North Platte River in October.

Raynolds continued south up the Bighorn then left the river for a southern course along the eastern flank of the Bighorn Mountains. He continued along this route through the Powder River Basin and reached the Oregon Trail and the upper Platte Bridge near Red Buttes in October. Reunited, the expedition established winter quarters on Deer Creek near the Indian Agency on the North Platte River between Fort Laramie and the lower Platte Bridge. Exploration would commence again in the spring of 1860.

Raynolds' route south from the Yellowstone River proved the feasibility of a wagon road through the region to the Oregon Trail and established the future route of the Bozeman Trail. His summary report said in part that "At the eastern base of the Big Horn mountains there is a belt of country some 20 miles in width that is peculiarly suitable for a wagon road, and which I doubt not will become the great line of travel into the valley of the Three Forks. Being at the base of the mountains, this strip is watered by . . . numerous streams. . . . a comparatively straight road can be laid out close to their foot . . . I traveled through this region with heavily loaded wagons in the fall of 1859 without embarrassment."

Raynolds' report also refers to Jim Bridger's superior knowledge of the Yellowstone country and along the eastern base of the Bighorn Mountains. "My American guide, Bridger is ... on familiar ground and appears to be entirely at home in this country."

The following spring, Raynolds and Lt. Maynadier twice traveled separate routes to increase the reconnaissance of the upper Yellowstone River region and the upper valley of the Bighorn River. Raynolds chose to explore the Yellowstone country. Maynadier's exploration along the upper Bighorn River through the Bighorn Basin provided the basis for future historians to claim that he was responsible for the location of the Bridger Trail route utilized by Bridger in 1864. Maynadier, however, had never been in the country before, and his guide was not nearly as experienced as Bridger. His route differed considerably from the trail blazed by Bridger four years later.

The difference between the two routes is apparent from a comparison of Raynolds' report and the emigrant diaries. Regarding the Bridger Trail as a whole, Maynadier did not traverse the portion between Red Buttes and Badwater Creek; he did not know of or travel the Bridger Creek/Kirby Creek route over the Bridger Mountains; he failed to find an adequate road once in the southernmost region of the Bighorn Basin; he followed a decidedly different route between the Greybull and Shoshone Rivers; and he followed a different route once along Clarks Fork to the Yellowstone River. Therefore, Jim Bridger is given the credit for locating the Bridger Trail route.