After leaving Clark at Traveler’s Rest, Meriwether Lewis proceeded north with his Nez Perce guides, and then took the East branch of Clark’s River (Clark Fork), probably camping near Grant Creek. The guides directed him to the river he should take--Cokahlarishkit, or "river of the road to the buffalo"--but they would not accompany them for fear of their enemies, the Minntares (Blackfeet).
On Friday, July 4, 1806 Lewis and nine men proceeded through Hellgate Canyon some five miles to the confluence of the Blackfoot and the Clark Fork River. Lewis described this section of the trip in his journal, writing, "…we then entered the mountains with the East fork of Clark's river through a narrow confined pass on it's N. side continuing up that river five ms. further to the entrance of the Cokahlahishkit R which falls in on the N. E. side, is 60 yds. wide deep and rapid. the banks bold not very high but never overflow. the East fork below its junction with this stream is 100 yds. wide and above it about 90. the water of boath are terbid but the East branch much the most so; their beds are composed of sand and gravel; the East fork possesses a large portion of the former. neither of those streams are navigable in consequence of the rapids and shoals which obstruct their currents. thus far a plain or untimbered country bordered the river which near the junction of these streams spread into a handsome level plain of no great extent; the hills were covered with long leafed pine and fir..."
For more on the Lewis & Clark Expedition, visit the National Park Service's Journey of Discovery website or this PBS website profile of Lewis.