In January of 1804 Thomas Jefferson sent Lewis a copy of John Evans's map.1 It provided the captains with valuable information about the Missouri River from the vicinity of today's Sioux City, Iowa, to the Mandan villages, where Evans concluded his exploration. It also contained a sketch of what the Indians at the latter place had told Evans about what lay beyond.2 The above excerpt focuses on the point at which the Missouri River emerges from among the four mountain ranges that were then thought to comprise the Rockies. During the winter of 1804-1805 at Fort Mandan, Lewis compiled an extensive catalog of what he and Clark had learned about the Missouri, its tributaries and its sources, and appended a summary of what they were told they would find west of the Knife River villages.3
They could expect to pass the mouth of the Meé,-ah'-zah, or Yellowstone River about 235 straight-line miles up the Missouri. One hundred twenty direct-line miles "nearly S.W. of the Yellowstone, the Mah-tush,-ah-zhah, or Muscle shell river falls in on the S. side." Actually, the latter distance is about 180 miles, and the direction is more nearly west-by-south, or only ten degrees south of west, an error that may have resulted from a minor misunderstanding about directions. In any case, the compass bearing was inconsequential since the explorers would be following the river wherever it went.4
Some of what Lewis and Clark heard from the Indians at Fort Mandan seemed to validate Evans's report. From their conversations that winter, Clark concluded the distance to the landmark waterfall would be roughly 610 miles.6 The rest added landmarks to help them measure their progress. All that remained was "ground-truthing."
1. Donald Jackson, ed., Letters of the Lewis and Clark Expedition with Related Documents, 1783-1854, 2nd ed., 2 vols. (Urbana: University of Illinois Press, 1978), 1:163.
2. W. Raymond Wood, "The John Evans 1796-97 Map of the Missouri River," Great Plains Quarterly, Vol. 1, No. 1 (Winter 1981), 39, 53. Jefferson's cover letter is in Jackson, Letters, 1:163.
3. Moulton, Journals, 3:362-367.
4. Elsewhere in Discovering Lewis & Clark, see "Indian Spatial Concepts," in Mapping Unknown Lands, by John Logan Allen. Also "Grasping the Northern Rockies." James P. Ronda, " 'A Chart in his Way': Indian Cartography and the Lewis and Clark Expedition," Great Plains Quarterly, 4 (Winter 1984), 43-53.
5. Actually, it is about 150 miles, "on a direct line," from the mouth of the Musselshell to the Great Fall. The following description is in Moulton, Journals, 3:367.
6. Ibid., 3:373-374.