In 1803, Meriwether Lewis began to prepare for an expedition across North America to the Pacific coast. He received guidance in scientific and medicinal practices from members of the American Philosophical Society in Philadelphia, and had a keelboat for river travel specially constructed in Pittsburgh. After travelling down the Ohio River, Lewis picked up William Clark in Indiana, and they wintered in Illinois, across the river from St. Louis, Missouri.
On May 14, 1804, following the spring thaw, the Corps of Discovery set off with more than 40 men, a dog named Seaman, and a reported 1,600 kilograms of supplies and equipment. They travelled along the Missouri River, following routes taken by traders and previous expeditions. The men passed the winter of 1804-05 at their Fort Mandan camp, in what is now North Dakota. The following spring, the expedition sent back its keelboat and proceeded up the Missouri with canoes. Anticipating the need for a translator to speak with the local peoples whose lands they were passing through, they hired a French-Canadian interpreter who travelled with his Shoshone wife, Sacagawea.
The expedition crossed the Rocky Mountains with Shoshone horses and guides and took the Clearwater, Snake, and Columbia Rivers to the Pacific. They built a camp at Fort Clatsop, Oregon, in December of 1805, where they survived the winter and prepared for their return journey. The Corps set off eastward on March 23, 1806; Lewis and Clark split up to cover more geography for their survey reports, although they followed essentially the same route as their westward travels. The expedition reunited in North Dakota and returned to St. Louis on September 23, 1806, to the surprise of many who believed they had perished after almost two and a half years in the distant west.