Headwaters of the Missouri - At Lemhi Pass in the Beaverhead Mountains, the Corps of Discovery found the spring that represented the headwaters of the Missouri River (A). At the crest of the Lemhi Pass the Corps would pass into the Pacific drainage and ¾ of a mile down the mountain Lewis would "taste from the water" of the great Columbia River (B). Their trail is marked by arrows. (Lewis, 8/12/1805)