From 1804-1806, Captains Meriwether Lewis and William Clark led the Corps of Discovery in a journey ordered by President Thomas Jefferson to find a water route to the Pacific Ocean through the newly acquired Louisiana territory. They mapped the land, recorded its resources, and met with and learned from its native inhabitants. In preparation, Lewis studied medicine, botany, zoology, and celestial observation, and with Jefferson's permission asked his friend and former commanding officer William Clark to serve as co-leader; Clark contributed skills as an outdoorsman, geographer, and map-maker, and despite opposite temperaments they worked harmoniously throughout the two-year journey. The expedition left Camp River Dubois on May 14, 1804, held a first council with the Otoes and Missourias on August 3, saw Sgt. Charles Floyd die on August 20, faced a confrontation with the Teton Sioux on September 25-28, and built Fort Mandan for the winter of 1804-1805, where Toussaint Charbonneau, Sacagawea, and their son Jean Bapiste joined the expedition. On June 2, 1805, Lewis and Clark correctly chose the Missouri River route toward the Great Falls, then made the portage around the falls from June 13-July 15, purchased horses from Shoshone Indians on August 30 to ensure safe passage through the Bitterroot Mountains, received help from the Nez Perce from September 26-October 7 in preparing for the journey to the Columbia River, and reached the Pacific Ocean on November 15, 1805. After constructing Fort Clatsop for the winter of 1805-1806, the return trip began on March 23, 1806; on July 3 the expedition split, with Lewis heading north to explore the Marias River and Clark heading south along the Yellowstone River. Clark engraved his name on a sandstone butte on July 25, Lewis's party fought at Two Medicine on July 27, resulting in the deaths of two Blackfeet Indians, the parties reunited on August 12, and they arrived in St. Louis on September 23, 1806. The landscape they crossed has since changed as rivers were dammed, forests cut over, prairies plowed under, and roads built, though remnants of wilderness still remain. Some Indian Nations who lived along the Missouri River during the expedition were later relocated to Oklahoma, including the Delaware, Kaw, Osage, Otoe-Missouria, Pawnee, Ponca, Potawatomi, and Shawnee.