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The ExpeditionIdaho: Lewis's River
Key Map, Lewis's River Valleys
A Great Height

A Gap in a Dividing Ridge

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lemhi pass

this photo of Lemhi Pass, marked by the convergence of both modern roads and traces of older travel ways, bears northwest (310° TN)1 toward the west (Idaho) side of the Continental Divide.2 The east (Montana) side is on the lower third of the picture. The area is now a National Historic Landmark under the jurisdiction of the Salmon-Challis and Beaverhead National Forests.

Lewis was still a half mile from the pass on August 12, 1805, when he arrived at the point he regarded as

the most distant fountain of the waters of the mighty Missouri3 in surch of which we have spent so many toilsome days and wristless nights.    thus far I had accomplished one of those great objects on which my mind has been unalterably fixed for many years, judge then of the pleasure I felt in allying my thirst with this pure and ice cold water.

After a few minutes' rest he and his three companions — George Drouillard, Private John Shields and Private Hugh McNeal — strode up to the pass where, instead of the Great River of the West flowing invitingly at their feet, they saw "immence ranges of high mountains still to the West of us with their tops partially covered with snow." Three-quarters of a mile down the western slope he paused at "a handsome bold running Creek of cold Clear water" where he "first tasted the water of the great Columbia river." It may have momentarily soothed his anxiety over the discouraging prospects, but we hear none of his previously exultant tone.

"immence ranges of high mountains
still to the West of us"

jenkinson photo

president Jefferson had specifically directed Lewis to "observations of latitude & longitude, at all remarkeable points." Now, the captains crossed Lemhi Pass a total of five times in August of 1805, yet not once did either of them mention the need to take observations, much less stop to set up the navigation instruments when they had them at hand on the final crossing. Here are the crossings in chronological order:

August 12, Westbound — Captain Lewis and 3 companions

August 15, Eastbound — Lewis and 3 men plus about 60 Shoshones on horseback

August 19, Westbound — Captain Clark, 11 men, Sacagawea, Charbonneau, five Indians

August 21, Eastbound — Sacagawea, Charbonneau, 50 Indian men plus some women and children, with many horses

August 26, Westbound — Lewis and the rest of the Corps, plus Cameahwait and other Indians. Baggage carried by 11 horses, 1 mule, and some Shoshone women. Sacagawea rode a horse. This was the only crossing on which they had their instruments with them.

Twice more, in four different places, the captains were to cross the Continental Divide. Clark would cross at today's Gibbons Pass (6941 ft) on July 6, 1806, en route back to Camp Fortunate, and Lewis would walk across Lewis and Clark Pass, 160 miles northeast of Lemhi Pass, on July 7. Both would cross Lost Trail Pass, 80 miles south of Travelers' Rest, that separated two major sources of the Columbia, the Snake River basin and the Clark Fork-Kootenai River basin. They also surmounted Lolo Pass, 30 miles west of Travelers' Rest, going to and coming from the Pacific.

They knew which of those "dividing ridges" separated the headwaters of the Missouri from those of the Columbia, and which did not. President Jefferson had made himself perfectly clear when it came to places as "remarkeable" as these: "The interesting points of the portage between the heads of the Missouri, & of the water offering the best communication with the Pacific ocean, should also be fixed by observations." Why didn't either of them ever take the time to follow the commander-in-chief's instructions regarding these particulars? That is likely to remain forever one of the foremost among the unanswerable questions left to us by Captains Lewis and Clark.

--Joseph Mussulman, 09/2009

1. The expression "310° TN" is the azimuth of this photograph. Azimuth (from an Arabic word meaning "the way") is an angular position on the horizon measured from a given point, which in this instance is designated as geographic or true north — TN. Lewis and Clark, in making observations to establish the bases for calculating longitude of a given place, used a circumferentor, or surveyor's compass, to determine the magnetic azimuth of certain celestial bodies. (See "Those Cryptic Journal Entries.") Today, aviators use azimuth to specify headings or directions of flight. The heading of this photograph is 310 degrees clockwise around a circle from true north, which in this part of the globe is currently (May, 2004) about 15.7° west of magnetic north, which is called magnetic declination. Compass bearings in Clark's daily "courses and distances" were specified east or west of north or south, with the shift made at due east or west. Jefferson had directed Lewis and Clark to note the "variations of the compass," and they did so to the best of their ability. Contemporary interpretations of Clarks compass bearings must therefore reckon with the comparative magnetic declination then. R. S. Preston, "The accuracy of the astronomical observations of Lewis and Clark." Proceedings of the American Philosophical Society, v. 144. no. 2, (2000): 168–191. Robert E. Criss, "Mid-continental magnetic declination: A 200-year record starting with Lewis and Clark," GSA Today, Vol. 13, No. 10 (2003), 4–11. The unique magnetic declination of any point on earth can be calculated through the resources of the National Geophysical Data Center at http://ngdc.noaa.gov/seg/geomag/jsp/struts/

2. The awareness of a more or less continuous ridge, or continental divide, separating the waters of the Gulf of Mexico from those flowing into the Pacific Ocean began to emerge in the 1820s. Part of John Jacob Astor's American Fur Company expedition, returning eastward from their trading post at the mouth of the Columbia, in 1812, discovered the low and easy South Pass, in western Wyoming, although little notice was taken of it at the time. The next to cross it was the American fur trader and explorer William Ashley (1778–1838), who evidently was aware of where he was when, at the end of March, 1825, he and his party "were employed in crossing the ridge which divides the waters of the Atlantic from those of the Pacific ocean." Lewis and Clark thus were aware only that they had reached the dividing ridge between the Missouri and Columbia River drainages, which was their main geographical discovery. In his "Course Distance & over the portage from the Waters of the Missouri to the Waters of the Columbia River" (Moulton, Journals, 5:177), Clark referred to Lemhi Pass merely as a gap in a "deviding mountain," or "deviding ridge." Harrison Clifford Dale, ed., The Ashley-Smith Explorations,...1822–1829, (Cleveland: Arthur H. Clark, 1918). John Logan Allen, Passage Through the Garden: Lewis and Clark and the Image of the American Northwest (Urbana: University of Illinois Press, 1975), 162–69.

3. The real "most distant fountain" of the Missouri River, which is approximately 90 miles southeast of Lemhi Pass, was not identified until 1895. In that year, geologist J. V. Brower pinpointed it as a small spring feeding Lillian Lake, in the Centennial Valley near the Idaho-Montana state line, west of Yellowstone National Park. The "River Mile Index of the Missouri River" (Helena: Montana Department of Natural Resources & Conservation, 1979), indicates the outlet of Lillian Lake is 2,625.7 miles from the mouth of the Missouri, and 131 river miles southeast of Horse Prairie Creek. J. V. Brower, The Missouri River and its Utmost Source (St. Paul, Minnesota: Pioneer Press,1897), 105–112.

Key Map, Lewis's River Valleys
A Great Height

From Discovering Lewis & Clark ®, http://www.lewis-clark.org © 1998-2014
by The Lewis and Clark Fort Mandan Foundation, Washburn, North Dakota.
Journal excerpts are from The Journals of the Lewis and Clark Expedition, edited by Gary E. Moulton
13 vols. (Lincoln: University of Nebraska Press, 1983-2001)