Documents their travels from the Three Forks of the Missouri River in present-day Montana and their progress over the rugged Bitterroot Mountains, along the nearly impenetrable Lolo Trail, and to the Cascades of the Columbia River on today's Washington-Oregon border.

President Thomas Jefferson conceived the Corps of Discovery to travel up the Missouri River to the Rocky Mountains and westward along possible river routes to the Pacific Ocean. Meriwether Lewis and William Clark led the expedition of 1804-6. Along the way they filled hundreds of notebook pages with observations of the geography, Indian tribes, and natural history of the trans-Mississippi West.