What was the task of Lewis and Clark?

Under the Louisiana Purchase of 1803, the US government bought a great deal of territory in central and western North America, but had insufficient knowledge about the lands themselves. US Army officers Meriwether Lewis and William Clark were sent by President Thomas Jefferson to explore it. They would study the terrain and the geology of the Missouri River valley, as well as the plants, wildlife, and indigenous Native American tribes. A less-publicized part of their mission was to study the activities and influence of Canadian trappers, who might have ties to the British or the French. Their "Corps of Discovery" also met along the way a French-Canadian fur trapper named Toussaint Charbonneau and his young Shoshone wife, Sacagawea. Both were then hired as guides and translators. The expedition had begun on August 30, 1803 in Pittsburgh PA with Lewis sailing supplies down the Ohio River. They crossed the Continental Divide and reached the Pacific Ocean on November 5, 1805, exploring the area around the Columbia River in present-day Oregon and Washington. They returned to St. Louis on September 23, 1806. Besides returning a large number of mineral and plant specimens, Lewis and Clark established a road map for the western expansion of the United States.