Lewis and Clark Expedition
Prior discoveries

In 1682, René-Robert Cavelier, Sieur de La Salle traveled down the Mississippi from the Great Lakes to the Gulf. The French then established a chain of posts along the Mississippi from New Orleans to the Great Lakes. There followed a number of French explorers including Pedro Vial and Pierre Antoine and Paul Mallet, among others. Vial may have preceded Lewis and Clark to Montana. In 1787, he gave a map of the upper Missouri River and locations of "territories transited by Pedro Vial" to Spanish authorities.

Early in 1792, the American explorer Robert Gray, sailing in the Columbia Rediviva, discovered the yet to be named Columbia River, named it after his ship and claimed it for the United States. Later in 1792, the Vancouver Expedition had learned of Gray's discovery and used his maps. Vancouver's expedition explored over 100 miles (160 km) up the Columbia, into the Columbia River Gorge. Lewis and Clark used the maps produced by these expeditions when they descended the lower Columbia to the Pacific coast.

From 1792 to 1793, Alexander Mackenzie had crossed North America from Quebec to the Pacific.
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