When Lewis and Clark first journeyed up the Missouri in the spring of 1804, trappers had already made their way up the river in search of beaver. Lewis and Clark encountered fur trappers as far north and west as the Mandan villages, where the Corps stayed their first winter. But by the time Lewis and Clark reached the mountains of Montana, they could easily have been the first white men to set foot on the Missouri's sandy banks.

The abundance of beaver Lewis and Clark reported seeing on their way up the Missouri River triggered a massive push west and an expansion of the fur trade. Beaver hats were continuing to grow in popularity, and as the demand grew, so did trapping along the Missouri.

Beaverhead Rock State Park (Click for a larger image)

But that was just the beginning. The gold rush followed the fur trade, and behind came homesteaders, who staked a claim of their own to Montana’s fertile valleys. Then came the sprawling cattle ranches, barbed wire, Indian reservations, incorporated towns, highways, airports, freeways—200 years of development and technology.

Not only did the Corps of Discovery ignite western expansion, it also secured the borders of the United States by mapping the new Louisiana Territory. Before Lewis and Clark, no reliable maps existed that positively identified the boundaries of the 820,000-square-mile territory. The Spanish, French and British were all searching for a Northwest Passage—and the country that could discover an all-water route between the Atlantic Ocean and the Pacific Ocean could control trade and the international economy. Each country had a different idea of the boundaries of the Louisiana Territory.

Clark’s detailed maps laid claim for the United States on the entire Louisiana Purchase. Without knowing it, he kept Billings from being a French city, Wyoming from being a part of Spain, and Idaho out of the hands of the British.