As challenging as the question of how Clark measured the falls of the Missouri may be, a more important question is Why? What compelled him to spend so much time and exert so much effort—even to risking his life—to go to such "great pains and accuracy," mixing rough guesses with calculations so compulsively precise as 87 feet, ¾ of an inch?
Clark's map of the Great Falls of the Columbia,
Engraving from History of an Expedition under the Command of Captains Lewis and Clark...(1814) Vol. 2, facing page 31.
Clark drew similarly detailed maps of both the Great Falls and the Cascades of the Columbia that, together with the Short and Long Narrows, made up a 55-mile stretch of falls, rapids and "shoots" that challenged the men's courage and endurance. But owing to the urgency of reaching the ocean before winter began, the crowds of people that lined the banks, and perhaps above all the apparent pointlessness of the exercise, he included only a few perfunctory measurements on those two maps.
First, he did it in conscientious obedience to an order from his commander-in-chief to take observations "with great pains & accuracy." The falls of the Missouri comprised the most remarkable of all the "remarkeable points" he described and mapped.1 Second, it was his obeisance to Enlightenment Science, his duty as a "philosopher," a "lover of knowledge." His job, like Lewis's, was to measure, to count, and to sum up all he saw—to recognize order in the wilderness. This exercise at the falls of the Missouri constituted Clark's most extensive single scientific observation. It was fully as meticulous as any of Lewis's botanical or biological descriptions, such as those of the camas and the grizzly bear.
Finally, his measurements outlined the full dimensions of his and Lewis's monumental misunderstanding of all the reports they had heard concerning "the fall" or "the cascade" of the Missouri.2 The 22 consecutive rapids and falls added up to 14-3/4 miles and 27 poles in extent; the aggregate height of the 22 consecutive rapids and falls came to 360 feet 2-3/4 inches, by Clark's estimate. As yet the explorers had no clear understanding of what barriers that lay ahead, but this experience made them fear the possibility of similar misapprehensions about the very word "waterfall" until the day they reached tidewater on the Columbia River. Meanwhile, for their generation at least, the Falls of the Missouri clearly were an insurmountable impediment to the usual modes of commerce. At this very time and place, dreamers were awakened from the centuries-long fantasy of a Northwest Passage.
1. Jefferson's orders to Lewis said nothing specifically about mapping, but emphasized celestial observations, which made mapping the only logical mode for placing those "remarkeable points" in meaningful relationships with each other. Specifically, the "remarkeable points" Jefferson mentioned were mouths of rivers, rapids, islands, "& other places & objects distinguished by such natural marks & characters of a durable kind, as that they may with certainty be recognised hereafter." 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), 1:61–62.
2. See John Logan Allen, Lewis and Clark and Image of the American Northwest (New York: Dover, 1975), 43–44, 245–247.