Stepping "Off the Map" in the Louisiana Territory

Stepping "Off the Map" in the Louisiana Territory. . The object of you mission is to explore the Missouri river and such principal stream of it, as by its course and communication with the waters of the Pacific ocean, whether the Columbia, Oregon, Colorado, or any other river may offer the most direct and practicable water communication across the continent for the purpose of commerce...Altho' your route will be along the channel of the Missouri, yet you will endeavor to inform yourself, by enquiry, of the character and extent of the country watered by it's branches... --- From President Thomas Jefferson's instructions to Captain Meriwether Lewis, June 20 1803 , The Lewis and Clark Expedition was the first major scientific venture funded by the United States of America. Mapping rivers was the principle stated goal of the Expediton. Water transportation was key to exploring the new territory's resources. At the beginning of the 19th Century, natural lakes and rivers provided only practical routes for moving major trade goods across a developing nation with almost no roads. , What geography did the explorers already know? , The Expedition carried a recent map that included two well-defined destinations on their route. The first was the Mandan Villages, where the Expedition built Fort Mandan as headquarters for the winter of 1804-05. The second was the mouth of the Columbia River, near which the Expediton built its 1805-1806 winter headquarters, Fort Clatsop. The Rocky Mountains and Black Hills were named, but only vaguely located. , At Fort Mandan, Lewis and Clark used the long winter to gather much new geographic information from Native Americans of several tribes. From them, the explorers gathered details of the Missouri and its tributaries all the way to the Great Falls. They learned Native American names for the major tributaries and the order in which they would be encountered. Their knowledge of the Missouri River and its tributaries was very much improved, but still very incomplete. , Unknowns , The biggest question was the nature of the Continental Divide separating the Missouri River drainage from the rivers that went to the Pacific. There were incorrect predictions that the Divide was a single broken chain of mountains with easy passes or just a flat pyramid-shaped high plain. Maybe most of the major rivers were just a short portage from each other...or maybe not. , The Expedition's contribution , Compare the 1803 Arrowsmith map and the 1814 Clark map to see how much the "Voyage of Discovery" added to geographical knowledge of the American West. , Would President Jefferson have made the Louisiana Purchase if he had known that a razor-backed bulge of many high mountain ranges, hundreds of miles wide in places, split the continent? As they passed the future site of Fort Union, that realization was still four months away for Lewis and Clark. , The Indians inform that the yellowstone river is navigable for perogues and canoes nearly to its source in the Rocky Mountains, and that in it's course near these mountains it passes within less than half a day's march to a navigable part of the Missouri. it's extreem sources are adjacent to those of the Missouri, river platte, and I think probably with some of the South branch of the Columbia river. -- Meriwether Lewis, April 26, 1805