Preparations

Forty-six or forty-seven people started on the expedition (the precise number is not known). The majority were soldiers (including a temporary detachment of seven), but the party also included boatmen, a hunter-interpreter, and Clark's black slave, York. The party assembled in the summer of 1802, and proceeded down the Ohio River and up the Mississippi to a point in Illinois opposite the mouth of the Missouri. The following winter was spent in preparation, with several trips to St. Louis, then a frontier furtrading village.

On March 9 and 10, 1803, Lewis was the chief official witness at the transfer of upper Louisiana to United States authority, under the terms of the Louisiana Purchase. This agreement brought under United States control all the territory the expedition would cross east of the Rockies. The Louisiana Purchase not only increased the importance of the expedition, but also eliminated the danger of foreign interference or objection.

Supplies gathered for the journey included 21 bales of goods for trade with the Indians. The boats for transporting men and supplies on the first stage of the expedition were a 55-foot (17-m) keelboat and two pirogues (large dugout canoes).