Clark refers to a list entered in Codex C of rivers and other geographical features above Fort Mandan, with estimated distances. The material is in [
Fort Mandan Miscellany]. The Mandans and Hidatsas would have been the only source for most of this. Clark listed tributaries of the Missouri and of the Yellowstone, and some mountain ranges, as far west as "a large river on the west of the mountain," probably a stream such as the Lemhi or the Salmon River in Idaho, or the Bitterroot or Clark's Fork in Montana, immediately beyond the continental divide. Distance estimates would have been based on the time it took the Hidatsas to travel to these points on horseback on their raids. Allen, 210–25, 241–50.