Detail from a map by Nicholas King (1803)1
In the spring of 1803, at the direction of Albert Gallatin, cartographer Nicholas King prepared a new map of the Northwest combining features selected from maps by Aaron Arrowsmith (1802), Andrew Ellicott (1796-1800), James Cook (1775), George Vancouver (1798), and others, each containing a more or less educated guess as to the location and general topography of the Northern Rocky Mountains and the rivers in their vicinity, which no white man had yet seen.
Thomas Jefferson and his circle of amateur geographers were under the impression that there were just two ranges of mountains in the West. The northern range, of which the southern end is shown here, were thought to end somewhere above the 45th parallel, and to be situated between 113° and 115° west longitude. The peaks named "The Heart," "The Pap," "Battle Hills" and "Bear's Tooth" were landmarks reported by Indians.
Conjecture also held that those "Stony Mountains" were separated from the mountains of "New Mexico" in the Southwest by a broad plateau. In fact, Jefferson specifically instructed Lewis to try to ascertain whether that plateau was mountainous or flat. Imagine Lewis's feelings when he reached the mountain saddle today known as Lemhi Pass on the Continental Divide, knowing he was in the vicinity of the 45th parallel, to find still more mountains, as far as his eye could see. There was Jefferson's answer!
At upper right is a cluster of details representing the villages of the Mandans and other Indian tribes in the vicinity of Knife River. Between them and the Stony Mountains are shown two branches of the Missouri River. The upper one is the proper Missouri; the lower corresponds to the Yellowstone River, which Clark would explore on the Expedition's return in the summer of 1806.
By the time he reached Lemhi Pass Lewis on August 12, 1805, Lewis was certainly aware how far King's map was off with regard to the sources of both branches of the Missouri. But when they descended into the valley to the west, he and Clark believed they had found the southern branch of the Columbia River that was represented on King's map by the dotted lines extending to the center from the left edge of the map above. No wonder they insisted on exploring the Salmon River – their "Lewis's River" – despite the Indians' warnings that it was impassable.
Reality was coming into focus.
John Logan Allen, Lewis and Clark and the Image of the American Northwest (New York: Dover, 1975), 97-108.
1. Map Division, Library of Congress.