Two and a half months after leaving Fort Mandan, Lewis, Clark and their crew found themselves at the base of the Great Falls of the Missouri. Hampered by rugged terrain, a carpet of prickly pear cacti, hail storms, hostile grizzlies and hordes of mosquitos, it would take them three weeks to complete portage around the 80 foot cascade.
By July 25, the explorers entered a broad valley hemmed in by mountains; it was here that they encountered river otter and sandhill cranes. At the valley's southern end were the Three Forks of the Missouri River which they named (from east to west) the Gallatin, Madison and Jefferson Rivers. Of the three, the Jefferson was largest and proved to be less turbulent; it also appeared to be rising from the Continental Divide and was thus chosen as the route to the Missouri headwaters (and to the Northwest Passage that they sought). Traveling upstream and westward along the Jefferson, they came to another fork, continuing southwestward along the larger stream (now known as the Beaverhead River).
As Captain Clark and his crew struggled against the current in canoes, Captain Lewis and two hunters traveled ahead on foot, eventually climbing to Lemhi Pass on the Continental Divide; the date was August 12, 1805 (15 months after leaving St. Louis). To the west they could see the mountain ranges and valleys of the Upper Columbia Watershed. With the assistance and advice of local Native Americans, Lewis and Clark mapped the route of their descent, reaching the Columbia River (at the mouth of the Snake) on October 16 and the Pacific Ocean one month later. The party returned to St. Louis on September 23, 1806 (a roundtrip journey of 2 years, 4 months and 9 days).