Stepping "Off the Map" in the Louisiana Territory

In instructions of June 20, 1803, President Thomas Jefferson directed Captain Meriwether Lewis to explore the Missouri River and its principal streams to find the most direct and practicable water communication across the continent to the Pacific for commerce, while also learning about the country watered by the Missouri's branches. The Lewis and Clark Expedition became the first major scientific venture funded by the United States of America, with mapping rivers as its principal stated goal at a time when lakes and rivers were the only practical routes for moving major trade goods across a developing nation with almost no roads. The expedition began with a recent map that showed two well-defined destinations, the Mandan Villages, where Fort Mandan was built for the winter of 1804-05, and the mouth of the Columbia River, near which Fort Clatsop was built for the winter of 1805-1806, while the Rocky Mountains and Black Hills were named but only vaguely located. During the winter at Fort Mandan, Lewis and Clark gathered much new geographic information from Native Americans of several tribes, learning details of the Missouri and its tributaries as far as the Great Falls, including the Native American names of major tributaries and the order in which they would be encountered, yet their knowledge remained incomplete. The greatest uncertainty was the Continental Divide between the Missouri drainage and the rivers flowing to the Pacific, which some incorrectly imagined as a single broken chain with easy passes or even a flat, pyramid-shaped high plain, leaving open the question of whether the major rivers lay only a short portage from one another. By the time the expedition passed the future site of Fort Union, the full realization that many high mountain ranges, hundreds of miles wide in places, split the continent was still four months away, even as Lewis recorded on April 26, 1805, Native reports that the Yellowstone River was navigable nearly to its source in the Rocky Mountains and passed close to a navigable part of the Missouri.

Trenton, North Dakota · United States