Lewis and Clark: Great Falls and Lemhi Pass

Background. The Corps of Discovery was commissioned by President Thomas Jefferson after the 1803 Louisiana Purchase to explore the newly acquired western territory. The expedition departed in May 1804 and wintered at Fort Mandan, where leaders learned of the Great Falls from the Mandan Indians. The Great Falls remained in Blackfeet tribal territory until American claims were established in 1803.

Great Falls of the Missouri. The Great Falls are a series of waterfalls on the upper Missouri River in north-central Montana. Lewis described them as the grandest sight he had beheld thus far in the journey. Four of the five waterfalls were named by Lewis and Clark in 1805. The falls lie atop the Great Falls Tectonic Zone, an intracontinental shear zone between geologic provinces.

Lemhi Pass and Continental Divide. Lemhi Pass marked the western border of the United States from the 1803 Louisiana Purchase until the 1846 Oregon Treaty. Meriwether Lewis crossed the Continental Divide at this pass on August 12, 1805, with three other expedition members. The entire expedition crossed the pass on August 26, 1805.

Pacific Ocean and Fort Clatsop. The expedition sighted the Pacific Ocean for the first time on November 7, 1805. In late November, the party voted to move their camp to the south side of the Columbia River near modern Astoria, Oregon, based on a recommendation of the local Clatsop Indians. Fort Clatsop served as their camp from December 8, 1805, until March 23, 1806.