North West Company fur hunters came to the Lemhi and Lost river country with Donald MacKenzie's annual Snake expedition in 1818, beginning two decades of trapping in that part of the country. From a base at Fort Nez Perces near the junction of the Snake and the Columbia, macKenzie covered a large region until his company merged with the Hudson's Bay Company in 1821. He went on to important new assignments, but Michel Bourdon brought his Snake expedition back from Spokane House to the Lemhi in 1822, discovering the valley around Challis that season. Finnan MacDonald followed in 1823, Alexander Ross in 1824, and Peter Skeene Ogden in 1825. In 1824, Jedediah Smith brought a small band of mountain men (based in St. Louis) into the area, which became a modest part of a broad zone of international rivalry for more than the next decade.
After extensive exploration of the Snake country from the Boise region to Green river, Bear river, and the Yellowstone, Donald MacKenzie decided to spend the winter of 1819-20 in a camp with a large band of Indians on Little Lost river. There he persuaded the local Shoshoni, Bannock, and Nez Perce to agree to peace among themselves so that they could join in the fur trade without military distraction. This peace conference brought long term stability to the Snake country fur trade, although the Bannock remained restless in response to white intrusion into their country. And Blackfoot raiders ranged into the Lemhi and Lost river region unchecked by any peace arrangement.
Until after the great Shoshoni smallpox epidemic of 1781 (which they unintentionally passed on to the Blackfeet), the Blackfeet had ranged far to the north and east in the plains. But after the Blackfeet acquired horses from the Shoshoni and guns from Canadian trappers, they expanded into Shoshoni domain. British fur traders identified the Indian trail through Lemhi pass as the great Blackfoot road. Shoshoni inhabitants and transient fur hunters all had to guard their horses and camps in the Lemhi-Lost river region from Blackfoot intruders, and children in the Hudson's Bay Company annual Snake expedition could have real sport frightening the entire brigade by yelling that the Blackfeet were coming. Finnan MacDonald, who had been fighting the Blackfeet since 1812, burned out a Blackfoot band during a battle along the Indian road on the upper Lemhi not far to the west of Lemhi pass in 1823. Altogether, the Blackfeet lost ten warriors in that confrontation and showed considerably more respect for British trappers after the incident. MacDonald--tired of his wars against the Blackfeet--decided that he would not return to the Snake country until beaver should have gold skins. So in 1824, Alexander Ross brought the Snake brigade back to Lemhi pass and explored upper Lost river along with the upper Salmon and adjacent regions. by the time he was through, his trappers had examined practically the entire Lost river-Lemhi Pass region. And while he camped October 14, on the upper Pahsimeroi, Jedediah Smith and six other unwelcome mountain men joined them. From then on, trappers from all directions took turns in depleting beaver resources of that part of the land.
Peter Skeene Ogden and Jedediah Smith set out from Flathead House with the Hudson's Bay Company Snake expedition late in 1824 and spent much of the winter in camps at various spots in Lemhi valley. That way he avoided Alexander Ross' error the previous winter of getting caught for weeks in deep snow in Ross' Hole along the upper Bitterroot. Jedediah Smith's party stayed with Ogden on the Lemhi from February 11 to March 19. Ogden had a hard time getting on south to Snake river, largely because lack of grass for his horses along Birch creek made that route impossible. So he finally crossed form the Lemhi to Little Lost river through a pass between Flatiron and Portland mountains used by Alexander Ross in the fall of 1834 for his return from the Challis area through Pahsimeroi valley to Bannock pass. (This same route presumably had been developed for Michel Bourdon or Finnan MacDonald a year or two before.) After getting his expedition in shape for a hard trip to Snake river, he left Little Lost river at the beginning of April on his way to Ogden's Hole not far from Salt Lake. By that time, Lemhi and Little Lost river valley beaver resources had been largely depleted. So in 1826, Ogden stayed farther west, and Jedediah Smith's followers also avoided the area.
Ogden returned to exploit the once fabulous trapping possibilities of the Boise-Payette region in the fall of 1827. Finding mountain men from the United States already at work there, he sent Thomas McKay to try Owyhee river while he headed for Little Lost river, taking the mountain men along with him to keep them out of McKay's way. This strategy worked fine, except McKay ran into delays. After crossing Lost river valley through Doublespring pass to Little Lost river in November, Ogden managed to get across Snake river valley to the Portneuf for his winter camp. When McKay came along, he went up from Little Lost river to the Pahsimeroi, which he descended to the Salmon. There he established a snug camp near a convenient buffalo herd. Ogden and McKay each tried three times to get in touch with each other before Francois Payette finally managed to get the two parties back in communication with each other the next spring. (Joseph Portneuf almost had succeeded in reaching McKay, but had to turn back on Little Lost river after breaking his rifle there and getting caught in deep snow.) When McKay's force reached Ogden's camp, May 6, 1828, British activity in the Lemhi-Lost river country ceased for two years. William Sublette brought a band of mountain men that way later in 1828, but lost Joseph Cote in a clash with the Blackfeet on Birch creek. After that, Birch creek was known as Cote's defile. Later in 1828, Thery Goddin (who explored Lost river--which the trappers called Goddin's river--a decade earlier while out with Donald MacKenzie) failed to survive a misadventure with the Blackfeet on Bear river, and the mountain men of the upper Snake country began to wonder how they could get some assurance of safety in that dangerous borderland.
John Work brought the Hudson's Bay Company Snake brigade back to the Lemhi during the fall of 1839. He hoped that by then that once valuable beaver domain would be restocked, but did not find enough to detain him very long. Coming up Lemhi valley to Birch creek, he headed up Pass creek and across a summit north of Saddle mountain so that he could descend to MacKenzie's 1819-1820 winter camp. This still remained the major landmark of that region. Ross had camped there June 2, 1824; Ogden, March 31, 1825 and November 11, 1827; now Work camped there on December 2, 1830. Mountain men came back to Little Lost river in June and August, 1831, followed by John Work again in 1832 on the Lemhi between Lemhi pass and Bannock pass. Captain B. L. E. Bonneville came along later in 1832 on his way to a winter camp on the Salmon about four miles below the Lemhi. In later years of the fur trade, though, the area attracted less interest because of earlier overtrapping. After 1834, a permanent post at Fort Hall provided a convenient trapper's base not too far away, and the later fur trade revolved around that center rather than depending so much upon mobile trappers' expeditions. By 1850, beaver prices had fallen so low that even the Indians no longer could be induced to participate in the fur trade, and beaver in the Lemhi and Lost river country no longer were subject to serious trapping pressure.