Lewis prepared supplies for the expedition. Clark turned his attention to finding the men they would need. The two agreed that they needed young, healthy, hard-working men who had experinece surviving in the wilderness. William had no doubt that he wanted York at his side.
The Lewis and Clark expedition--formally referred to as the Corps of Discovery--set out on May 14, 1804 by boat up the Missouri River. York was among the four dozen men who rowed the Corp's large keelboat and two smaller flat-bottomed pirogues. Food, clothing, tools, weapons, gunpowder, and lead for bullets, along with bundles of beads, scissors, combs, knives and other items that could be used in trade with Indians--loaded down the boats.
The Missouri River was filled with dangers, causing the boatmen to have to avoid branches and sometimes whole uprooted trees swept downstream by the rapid current. At times, York and the others struggled to free a boat that had come stuck on a sandbar. The crew worked long hours every day, using oars, long poles, and towropes to travel ten miles upstream.