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 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 share leadership; Clark contributed skills as an outdoorsman, geographer, and map-maker, and despite opposite temperaments the two worked harmoniously throughout the two-year journey. Leaving Camp River Dubois on May 14, 1804, the expedition held its first council with the Otoes and Missourias on August 3, 1804, saw Sergeant Charles Floyd die on August 20, 1804, and faced a confrontation with the Teton Sioux on September 25-28, 1804. At Fort Mandan, their winter quarters in 1804-1805, 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 Great Falls from June 13 to July 15, 1805. After purchasing horses from Shoshone Indians on August 30, 1805, they secured safe passage through the Bitterroot Mountains, and with help from the Nez Perce from September 26 to October 7, 1805, prepared for the journey to the Columbia River before arriving at the Pacific Ocean on November 15, 1805. They built Fort Clatsop as winter quarters in 1805-1806 and began the return trip on March 23, 1806. On July 3, 1806, the expedition split, with Lewis and his party heading north to explore the Marias River and Clark and his party heading south along the Yellowstone River; on July 25, 1806, Clark engraved his name on a sandstone butte along the Yellowstone River, and on July 27, 1806, a clash at the Two Medicine Fight Site resulted in the deaths of two Blackfeet Indians. The parties rejoined on August 12, 1806, and arrived in Saint Louis on September 23, 1806. Since their exploration, rivers have been dammed, forests cut over, prairies plowed under, and roads built across the landscape, though some 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.