From 1804 to 1806, Captains Meriwether Lewis and William Clark led the Corps of Discovery on 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 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 the two 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, 1804, saw Sgt. Charles Floyd die on August 20, 1804, and faced a confrontation with the Teton Sioux on September 25-28, 1804. Fort Mandan was built for the winter of 1804-1805, and Toussaint Charbonneau, Sacagawea, and their son Jean Baptiste joined the expedition there. On June 2, 1805, Lewis and Clark correctly chose the Missouri River route toward the Great Falls, then made the portage around the Great Falls from June 13 to July 15, 1805. They bought horses from Shoshone Indians on August 30, 1805, helping secure safe passage through the Bitterroot Mountains, and from September 26 to October 7, 1805, the Nez Perce helped them prepare for travel to the Columbia River. The party reached the Pacific Ocean on November 15, 1805, built Fort Clatsop for the winter of 1805-1806, and began the return trip on March 23, 1806. The expedition split on July 3, 1806, 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 along the Yellowstone River on July 25, 1806, and on July 27, 1806, a clash at Two Medicine Fight Site led to the deaths of two Blackfeet Indians. The Lewis and Clark parties reunited on August 12, 1806, and arrived in St. Louis on September 23, 1806. The landscape they explored has since been altered by dams, logging, plowing, and roads, though remnants of wilderness remain, and 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.