by John L. Allen
(This article first appeared in Montana [The Magazine of Western History] 21.3 : 2–17.)
On April 7, 1805, The Corps of Western Discovery under the command of Meriwether Lewis and William Clark left winter quarters near the fortified villages of the Mandan nation in present-day North Dakota and, in six canoes and two pirogues, headed up the Missouri River into the unknown—"a country at least two thousand miles in width, on which the foot of civilized man had never trodden." 
Like virtually all exploring expeditions, this one had a goal—a goal as old as Europe's realization that the New World was a barrier lying athwart the all-water route to the Orient. For three centuries, the conjectural Northwest Passage or Passage to India had conditioned North American exploration. The experiences of explorers during those centuries had altered the nature of the dream passage and the earliest conceptions of a sea-level strait across the continent gave way to views of inland seas connected to the eastern and western oceans by mighty rivers.
These rivers retreated, in turn, before ideas of great navigable waterways which mingled their waters somewhere in the interior of the continent and provided interconnected water travel from east to west.
But although exploration had changed the nature of the conceptual Passage, it had not changed the dream itself. By the time of Lewis and Clark, the Passage to India had been articulated in its final and most realistic form—it was a simple and very short portage between the headwaters of a navigable river which flowed to the Atlantic and the sources of a great river draining toward the setting sun. Guided by Thomas Jefferson's specific instructions to locate the portage between the heads of the Missouri and Columbia rivers and establish thereby the "most direct and practicable water communication across this continent for the purposes of commerce,"  the men of the Lewis and Clark Expedition turned their faces toward the West and toward the final solution to the centuries–old riddle of the water route across North America.
Preparation for this attempt on the Northwest Passage had been lengthy and careful; among all the planning efforts, none were more painstaking or more important than the accumulation of the then-available geographical knowledge about the western parts of North America. Thomas Jefferson, conceiver and sponsor of the Expedition, had spent years gathering and analyzing geographical and cartographic information on the possible connections between the Missouri and the first-mythical-then-actual Great River of the West or Columbia. 
Much of this information had been imparted to Meriwether Lewis, Jefferson's personal secretary, prior to the Expedition. Before the party left Fort Mandan in the spring of 1805, considerably more lore on western geography had been added.
Lewis and his co-commander, William Clark, had, during the encampment at Wood River near St. Louis in the winter of 1803–04, studied the maps and journals brought from Washington by Lewis or forwarded by Jefferson and others. This material was combined with data obtained from merchants, fur traders, and government officials in the St. Louis area. When the captains started their journey to the Pacific in the spring of 1804, they took with them a set of definite geographical conceptions about the West, gleaned from the materials they had spent months in studying.
Although neither captain wrote a description or drew a map to illustrate their image of the West before leaving Wood River, it is possible to understand the nature of their views on western geography by examining those geographical materials which they considered to be the most important. Among all the items upon which Lewis and Clark relied most heavily in making their plans, none was more representative of the extent and character of their geographical knowledge than the 1802 edition of Aaron Arrowsmith's map of North America.
This map, drawn by a British cartographer and based on the late Eighteenth Century surveys of Peter Fidler, a Hudson's Bay Company employee, was widely considered by the finest geographical minds of the time to be the best and most accurate representation of the West. As such, it is symbolic of the nature of the image held by Lewis and Clark. From it, and from other materials which supplemented it, the captains derived those elements of their image which bore directly on their central objective of locating the shortest water route across the continent. 
In the explorers' mind's-eye view of the country through which they were to pass, the significant features were the drainage systems of the Missouri and Columbia rivers, the courses and navigability of those streams, and the Rocky Mountain source region from which they flowed. And in the image held by Lewis and Clark in the spring of 1804 these elements held the promise of the Passage to India.
The drainage system of the Missouri was seen as triangular or fanlike in shape. The base of the triangle lay along the eastern side of a range of mountains where, between the 45th and 50th parallels of latitude, a number of small but navigable rivers had their sources. These rivers flowed toward the east and converged to form two major streams which joined at the apex of the triangle near the Mandan villages, recognized by Lewis and Clark as the furthest penetration of explorers and traders into the western interior of the Louisiana Territory. From the villages, the river that was now called the Missouri turned toward the southeast to enter the Mississippi at St. Louis.
Of the two main rivers that joined near the Mandan villages, the southern was the true Missouri. It had its source in the mountains near the headwaters of streams which flowed toward the Pacific. From this source it ran a course that was nearly due west-to-east and was navigable throughout. This was the proper route to the Pacific. 
The mountain range in which the Missouri had its source appeared in the captains' image as a narrow, single-ridge structure, extending from south to north and being extensive in neither height nor breadth. On the western slope of this range, opposite the heads of the Missouri and the streams which fed it, were the source waters of the Columbia system. Although the Columbia was more of a geographical unknown than the Missouri, it was assumed that its source rivers were also navigable to their heads.
The assumed navigability of the upper reaches of both the Missouri and the Columbia combined with the captains' notions on the size of the mountain range they knew as the Rocky or Stony Mountains to create the core of misconception upon which their mission was based. The Missouri was navigable to its source in a range of low mountains and only a short and easy portage across that range would be necessary to link the waters of the Atlantic with some navigable branch of the Columbia. With this linking the ancient dream of the Passage to India could become a reality.
The image of western geography that Lewis and Clark carried up the Missouri in the spring of 1801 underwent significant changes during the first summer of exploration and the winter hibernation at Fort Mandan. Acquisition of geographical knowledge did not cease with the Expedition's departure from the Wood River camp and inputs of new knowledge from documents and maps, fur traders, and Indians served to restructure the captains' geographical understanding of the West.
Even before leaving Wood River, Lewis and Clark acquired documents from the explorations of John Evans and James Mackay, Spanish-sponsored explorers in the Missouri Valley during the late Eighteenth Century.  A journal written by Mackay and a map of the country west of the Mandan villages, drawn by Evans, provided the captains with the most current knowledge available on the West. Although this data was received too late to become assimilated into their image before their departure, Lewis and Clark would find it vital for later modifications of that image.
Even more important was information received during the winter at Fort Mandan. Throughout the long winter there, from November of 1801 to April of 1805, Lewis and Clark were visited in their winter quarters by Mandan and Minitaree Indians and by traders in the employ of the British Northwest Company. From these visitors the captains obtained sketches and oral information on the country to the west.  This data, in conjunction with the Mackay–Evans material, was responsible for changes in the earlier image. New information had given old ideas new expression and although the central theme in the explorers' image of the West was still the Passage to India, many of the elements within that image had undergone change by the end of the winter at Fort Mandan.
The nature of these changes and the character of the new image can be seen quite clearly in what is, quite possibly, the most important documentary material of the entire Expedition. This material, compiled by the captains during the Fort Mandan winter, includes Lewis's lengthy and detailed geographical description of the West and Clark's large map of the country from St. Louis to the Pacific. 
One of the major components of the new image was the recognition that neither the straight-line course of the Missouri from the mountains to the Mandan villages nor the fanlike drainage pattern of the Missouri system as shown on the Arrowsmith map were accurate.
From the Mackay–Evans material and from the Indian information, Lewis and Clark had learned that the Missouri did not divide into two major branches above the Mandan villages. Instead, the Missouri proper was the only major river west of the Mandans. It did, it is true, flow a fairly straight course from the mountains. But its sources were far to the south and it flowed northward between several mountain ranges before bending toward the east and passing out of the mountains.
This was a critical restructuring of the view of the West. Lewis and Clark had learned of the long south-to-north passage of the Missouri through the mountains. They had also learned that the Rockies were not a single ridge but were composed of several ranges. And, finally, they had learned of a critical landmark of the Upper Missouri. At the point where the Missouri left the mountains and flowed into the plains there was a great cataract. This was the Missouri's Great Falls and both the Mackay–Evans data and the Indian information agreed on its location near the Missouri's emergence from the Rockies.
Since their sources of data placed the Falls near the 47th parallel of latitude, Lewis and Clark concluded that the fanlike drainage system shown on the Arrowsmith map between the 4th and 50th parallels was a misrepresentation. Arrowsmith's data provider, Peter Fidler, could not have gone as far south as the 45th parallel without seeing the Missouri or its Great Falls. Therefore, his surveys must have all been made north of the Missouri and the mountains he saw with small streams issuing from them must lie above the 47th parallel. 
These corrections in the Arrowsmith map were not the only changes made in the captains' image of the West. Another feature of the new image was an absolute increase in the quantity and quality of knowledge of the country between the Mandan villages and the Pacific. This new knowledge included vital descriptions of the Missouri itself, of the streams tributary to it, of its passage through the Rockies, of the portage between its source and the waters of the Columbia, and, finally, of the course of the Columbia to the Pacific.
Above the Mandan villages, the Missouri ran a direct course to the Great Falls, located near the first or easternmost range of the Rockies. Between the villages and the mountains the Missouri was joined by the Knife, the Little Missouri, the Yellowstone, and the Musselshell, all coming from the south. From the north came the White Earth River and the Missouri's most important northern tributary, a river that the Indians had called "the river which scolds at all others."
From the Falls, the Missouri turned to the southwest and was joined from the west by the Medicine River. The southwesterly direction continued through a second and third range of mountains to a point above the third range of mountains where three rivers of nearly equal size joined to form the Missouri proper. Here was knowledge of another critical landmark—the Three Forks.
The westernmost fork of the Missouri above Three Forks was navigable to the foot of the fourth and final range of the Rockies. Across this range to the west, at a distance of a half-day's journey, the waters of a mighty southern branch of the Columbia ran in a south-to-north direction and then, bending westward, flowed through plains to the Columbia proper and thence to the Pacific. 
Here was the short portage, the Passage to India. It did exist and therefore it could be located and negotiated and all the commerce of the Orient could be brought to American shores. Full of the hope and expectation and optimism that this image created, the Expedition left its winter camp.
The upriver voyage from Fort Mandan, begun on April 7, 1805, proceeded past the mouths of the Little Missouri, White Earth and Yellowstone rivers, each located approximately where it should have been, according to the available knowledge. This part of the journey was pleasant and without major incident. Although the wind blew violently from the west and northwest, stinging the faces of the men and sometimes obscuring the far shores of the Missouri with clouds of dust and sand, the countryside was rich and beautiful, grass-cloaked and teeming with immense herds of buffalo, deer, elk, and antelope. It was a beautiful spring and if the winds continued to blow and ice formed on the cooking pots during the nights, the days were warm. By May 4, the snow had all but disappeared and the open grasslands and trees along the river were showing the light green of spring in the Great Plains.
On May 8, the Expedition reached the mouth of the river that they named the Milk, after the color of its waters. The captains concluded that this river was the one the Indians who had visited Fort Mandan during the winter had called "the river which scolds at all others." The Milk River was the only major tributary entering the Missouri from the north, according to their Indian informants. As such, it was an important landmark and to find it about where they had anticipated meant that the conceptions with which they left Fort Mandan were holding true.  Above it lay the Great Falls, the Three Forks, and the short portage to Columbian waters.
For the next ten days the small fleet sailed through the fantastic country of the Missouri Breaks. Although Lewis and Clark believed that the Great Falls were at least two hundred miles from the mouth of the Milk Rivera,  both they and the men became extremely anxious to come in sight of the first ranges of the Rockies that their information had represented to them as beginning near the cataracts of the Missouri.
They were temporarily elated when, on May 19, Clark climbed a hill on the northern side of the Missouri and saw to the west and north the first outlying ranges east of the Rocky Mountains. These mountains remained in sight on both sides of the river for several days. They did not match the descriptions given them by the natives, however, and the captains soon realized that they were not viewing the ranges near the Falls. 
From the first sighting of the scattered ranges east of the Rockies, the progress of the Expedition up the Missouri became more tedious. For hours at a time the party was compelled to beach their craft, waiting for the wind to abate. The river became more and more choked with shoals and sandbars and the captains must have begun to wonder about the validity of their assumptions about the navigability of the Missouri. In most places the boats had to be towed by lines from the shore. The rocks of the bluffs, broken here and there by the numerous dry channels through which water from the plains above the Missouri enters the river, closed in on either side and made towing a hazardous and difficult operation.
Mountainous country continued to be visible north and south of the river and when, on May 26, the very tips of the Highwood Mountains came into view just above the western horizon, the captains were convinced that they were nearing the first range of the Rockies and, therefore, the Great Falls.  Traveling became easier above the unexpected Judith River and the distant mountains remained in sight. Anticipation now ran high. Lewis and Clark knew, from their distance calculations and the proximity of the mountains, that they were close to the Falls and to the portage of hope and desire which lay beyond.
But before reaching the Great Falls, the Lewis and Clark Expedition encountered one of the greatest obstacles in the course of their two-year journey, a problem that they would spend more than a week in solving completely. In the late afternoon of June 2, the party came to and camped in a grove of small cottonwoods on the Missouri's left bank, opposite the mouth of a "very considerable river" which entered the Missouri from the north and almost equalled it in size.
This river was the Marias and its presence was puzzling. In the explorers' store of geographical information there was agreement that the Missouri had only one major northern tributary. That was "the river which scolds at all others" and they had passed the river fitting that description more than three weeks before.
What, then, was to be made of this second northern river? Or, if the northern branch were the true Missouri, what was to be made of the southern branch? Their information contained no account of a large southern tributary stream above the mouth of the Yellowstone. 
The true Missouri was the river, according to their conceptions of western geography, that had its sources within portaging distance of the navigable waters of the Columbia River. It was, therefore, the river that must be followed to its uppermost reaches if the Passage to India were to be found. But the unexpected division of the Missouri into two streams of near-equal size necessitated a major decision: "which of these rivers was the Missouri, or that river which the Minetares call Amahte Arz-zha or Missouri, and which they had described to us as approaching very near to the Columbia River?" 
For the purpose of answering this question, the party crossed the Missouri below the junction early on the morning of June 3 and "formed a camp on the point formed by the junction of the two large rivers."  Here the decision that would determine the success or failure of the entire enterprise would be made:
to mistake the stream at this period of the season, two months of the traveling season having now elapsed, and to ascend such stream to the rocky Mountain or perhaps much further before we could inform ourselves whether it did approach the Columbia or not, and then be obliged to return and take the other stream would not only loose us the whole of this season but would probably so dishearten the party that it might defeat the expedition altogether. convinced we were that the utmost circumspection and caution was necessary in deciding on the stream to be taken. 
The process of making this critical decision was not, as it has often been supposed, a simple one based on field observation alone. Nor was the decision itself just a lucky guess. Instead, the decision-making procedure was a complex and well–calculated series of operations lasting more than a week.
On the first day at the junction, the captains decided to make a preliminary investigation of the width, depth, and speed of flow of the two rivers in order to determine which was the major stream. To this end they sent a light canoe manned by three men up each branch. In an attempt to get some kind of long-range bearing on the direction from which the two forks came, they also sent several small parties by land to travel as far as possible and still return by nightfall.
Meanwhile, Lewis and Clark remained at the camp between the two rivers and made observations in the immediate area. During the morning they strolled to the highest ground between the Missouri and Marias and from this observation point had what Lewis termed an "inchanting view:"
to the South we saw a range of lofty mountains which we supposed to be a continuation of the S. Mountains. stretching themselves from S.E. to N.W. terminating abbruptly about S.West from us; these were partially covered with snow; behind these Mountains and at a great distance, a second and more lofty range of mountains appeared to stretch across the country in the same direction with the others, reaching from West, to the N. of N.W., where their snowey tops lost themselves beneath the horizon. This last range was perfectly covered with snow. 
But although the view was sublime, it was inconclusive. Little could be learned about the courses of the rivers, their channels being obscured by the convolutions of the countryside.
From the high ground the captains wandered down into the lovely little valley of the Teton River, through chokecherry and gooseberry and wild rose, thence back to the junction of the Missouri and Marias. Here they examined the nature of the two main rivers more closely:
we took the width of the two rivers, found the left hand or S. fork 372 yards and the N. fork 200. The no[r]th fork is deeper than the other but it's currant not so swift; it's waters run in the same boiling and roling manner which has uniformly characterized the Missouri throughout it's whole course so far; it's graters are of a whitish brown colour very thick and terbid, also characteristic of the Missouri; while the South fork is perfectly transparent runds very rappid but with a smoth unriffled surface it's bottom composed of round and flat smooth stones like most rivers issuing from a mountainous country. the bed of the N. fork composed of some gravel but principally mud. 
“In short,” wrote Lewis, "the air and character of this river [the Marias] is so precisely that of the Missouri below that the party with very few exceptions have already pronounced the N. fork to be the Missouri."
But at this point, sometime in the afternoon of June 3, Lewis and Clark showed their competence as field observers and analyzers of geographical information and made the initial assessment that further study would prove to be correct.
“If we were to give our opinions I believe we should be in the minority," noted Lewis. It is quite apparent that, in their considered opinion, the southern river was the one the Indians had described to them as approaching the waters of the Columbia. The limited and brief reconnaissance undertaken that morning was responsible for this tentative conclusion. But the conclusion was possible only because the things observed during the mornings' field work were analyzed in the light of what were, to Lewis and Clark, the "known" graphical facts.
When the captains returned from their reconnaissance of the morning of June 3, they returned with a perspective that was crucial for their decision. From the height-of-land separating the Missouri and Marias they had not been able to see the course of either river beyond the junction—but they had seen mountain ranges to the south and southwest. The nearest range of mountains had been in view for over a week and were not, by this time, considered to be the first range of the Rockies that was believed to begin near the Great Falls.
But behind these ranges, to the south and southwest and at a great distance, lofty, snow-covered peaks were visible and these might well have seemed to be the range near the Falls. If so, and if the southern branch continued its southwesterly trend above the junction, then it was the river which lay in the proper geographic relationship with the mountains and was, consequently, the river leading to Columbian waters.
Furthermore, their examination of the characteristics of the rivers upon their return to the base camp at the junction had shown the waters of the southern fork to be transparent, running over a bottom of round and flat smooth stones. It was "like most rivers issuing from a mountainous country" and the true Missouri, according to the captains' image, ran through mountains for a considerable distance from its source, all the way to its entrance into the plains at the Great Falls. The northern fork, their examination showed, was a muddy and silty river and although it was very similar to the Missouri below the junction, Lewis and Clark were inclined to view it only as a tributary stream. It might well have been part of the drainage system indicated in their re-evaluation of the Arrowsmith map and probably passed through the plains north of the Missouri without penetrating the mountains far enough to interlock with the Columbia.
On the evening of June 3, Lewis wrote:
I am confident that this river rises in and passes a great distance through an open plain country ... convinced I am that if it penetrated the Rocky Mountains to any great distance it's waters would be clearer unless it should run an immence distance indeed after leaving those mountains through these level plains in order to acquire it's turbid hue. 
The river whose transparent waters flowed from the southwest, from the mountains, was probably the Missouri and, therefore, the proper route to the Pacific.
But this assumption was only tentative. The parties that had been sent by land and water up both branches had returned with no conclusive information and the captains were still puzzled by the failure of their Indian informants to mention the junction of the Missouri and a large tributary stream.  Furthermore, it is apparent from the tone of the captains' journal for June 3, that they were the only members of the party who did not adhere to the belief that the northern fork, because of its similarities with the Missouri below, was the proper river to follow.
Two factors played a role in shaping the events of the week to follow. First. Lewis and Clark were good enough officers to realize the potential deterioration of their command situation should they go against the opinions of their men—some of whom were trained rivermen and wilderness experts—and then be proved wrong. And second, although the captains might have been firm in the belief that the left-hand fork was the Missouri and had the geographical evidence to support this view, they were too competent not to recognize the tactical dangers of a hasty decision without more definite proof of their preliminary assessment. Accordingly, it was concluded that each officer should take a small party and "ascend these rivers untill we could perfectly satisfy ourselves of the one, which it would be most expedient for us to take on our main journey to the Pacific." 
On the cool and cloudy morning of June 4, Captains Lewis and Clark departed from the camp at the junction of the Missouri and Marias rivers. Clark and a party of five men set out up the right-hand side of the Missouri, keeping to the higher lands well back from the river in an attempt to get the best views possible. By the end of a rainy afternoon they had traveled nearly 30 miles and made camp in an abandoned Indian shelter somewhere near the later site of Fort Benton. Except for an encounter with a couple of grizzlies, the small party spent an uneventful night alongside the Missouri, the waters of which continued to run rapidly over a gravelly bed.
It rained and snowed intermittently throughout the night and when the party prepared to break camp the morning of the 5th, they noted considerable amounts of snow on the mountains (the Highwood Mountains) southeast of their campsite. By around noon they had come in sight of other snow-covered mountains (the Little Belt and Big Belt ranges) to the southwest and Clark had seen enough to convince him that the river they were following was the true Missouri. From a ridge high above the river Clark could see the waters of the Missouri, still running deep and swift and trending toward the southwest, toward the mountains that he assumed to be those near the Falls.
To proceed any further would be useless and Clark and his men struck out overland for the main camp at the junction, reaching the Teton River in the late afternoon and making camp in its valley. At five o'clock on the next afternoon, June 6, the party led by Clark arrived at the junction of the Missouri and Marias where they expected to find Lewis waiting for them.
But Lewis was taking a little bit longer to reach a firm conclusion. He and his party of six men had crossed the Marias from the camp at the junction on the morning of the 4th and had proceeded on foot up that river along its right bank. At a distance of about five miles from the main camp, Lewis climbed a hill from which he viewed the "North Mountains" (the Bearpaws) lying toward the northeast and saw the northwesterly trend of the Marias. To the northwest, along the course of the river he was following, he could see nothing but a range of hills and since this view was inconclusive, he determined to proceed further.
The river continued its course to the north and west and for the remainder of the day, Lewis and his party traveled through the plains behind the river bluffs and, when the ravines grew too steep and numerous, along the bottomlands of the Marias. The evening encampment was made amidst clumps of willows which provided protection against the wind but did little to keep out the rain which continued to fall most of the night.
The men awoke cold and wet on the morning of the 5th and broke camp early, hoping to keep warmer by walking. The river kept to its north by west direction and before noon had led the party to a site from which they could see a high mountain (the main peak of the Sweetgrass Hills) toward the northwest and at a great distance. Late afternoon brought them in sight of still more mountains (the other peaks of the Sweetgrass Hills) in the northwest and here they made camp in a grove of cottonwoods and the ever-inquisitive Lewis experimented by roasting some prairie dogs for supper and found them "well flavored and tender. 
Had it not been for his systematic method of observation and analysis, the view of the mountains to the northwest could have led Lewis to the conclusion that this range was the one represented as starting near the Great Falls and that this northern river, therefore. was the Missouri. But the mountains were, Lewis believed, at least 80 or 100 miles away. This was too distant to have been the ranges near the Falls, according to the captains' mileage calculations not a very great distance above the junction on whichever river was the true Missouri. Moreover, the course of the river that Lewis was following ran so far north that it, in his thinking, must have drained a vast plains area and be part of the northern waters of the Missouri as shown on his and Clark's reinterpretations of the data from the Arrowsmith map. By the morning of the 6th Lewis had become "well convinced that this branch of the Missouri had its direction too much to the North for our rout to the Pacific"  and decided to return to the main camp.
While Lewis and four of his men engaged themselves in constructing rafts to descend the Marias, two others traveled farther up the river in order to get a more precise bearing on its course. They returned around noon with the report that the river did continue its northerly course as far as they could see and, after lunching on elk killed the night before, the small force embarked on their two hastily-constructed rafts to the mouth of the Marias. But attempted navigation of the Marias was unsuccessful and the rafts were soon abandoned for a less comfortable but more secure land route. This took them along the river bottoms where possible and across the exposed plains that offered little protection from the wind and rain which began soon after they left the rafts and continued through the afternoon and evening.
After a night spent in an unsheltered spot, Lewis and his cold, exhausted men broke camp early and resumed their trek across plains grown slippery and treacherous from the prolonged rains. On the evening of the 7th they bivouaced comfortably in an old Indian shelter. They resumed the return journey to the main camp on the cloudy and cool morning of June 8. By ten o'clock that morning the clouds had broken under a warming spring sun. As the weather improved so did Lewis's spirits as the party passed through "one of the most beautifully picturesque countries I ever beheld." 
But the captain had little else to be enthusiastic about. For although he had fully concluded that this river was "neither the main stream, nor that which it would be advisable for us to take" and gave it the name "Maria's River," the whole of his party was "fully persuaded that this river was the Missouri."  This was a potentially serious problem and one that he and Clark would have to work out when, at five o'clock on the afternoon of the 8th, the two captains were reunited at the junction of the Missouri and the river that was now officially the Marias.
While Lewis and his men relaxed with a "drink of grog" during the evening of the 8th, Clark began plotting the courses of the two rivers as far as he and Lewis had ascended them. Looking at the crude charts, both captains were more convinced than ever that their initial supposition about which stream was the Missouri was correct. They also came to the conclusion that they had been justified in making corrections in the Arrowsmith map during the winter at Fort Mandan. In his journal for June 8, Lewis wrote:
I now began more than ever to suspect the varacity of Mr. Fidler or the correctness of his instruments ... we are now within a hundred miles of the Rocky Mountains, and I find from my observations of the 3rd. Inst [June 3] that the latitude of this place is 47°.24'12".8. the river must therefore turn much to the South between this place and the rocky Mountains to have permitted Mr. Fidler to have passed along the Eastern border of these mountains as far S. as nearly 45° with-out even seeing it. but from hence as far as Capt. C. had ascended the S. fork or Missouri being the distance of 55 (45 miles in straight line) Miles it's course is S. 29° W. and it still appeared to bear considerably to the W. of South as for as he could see it. I think therefore that we shall find that the Missouri enters the rocky mountains to the North of 45°. 
Analysis of the geographical data derived from all sources, including the captains' own separate field reconnaissances, continued on the 9th and further settled in the minds of Lewis and Clark "the propryety of addopting the South fork for the Missouri, as that which it would be expedient for us to take."  The captains determined that the Arrowsmith map, incorrect as it was, provided a strong argument against the north branch or Marias as the true Missouri. Even if Arrowsmith's informant, Fidler, had penetrated as far south as 47' and had seen only small streams running east, then the presumption was that "those little streams do not penetrate the rocky mountains to such distance as would afford rational grownds for a conjecture that they had their sources near any navigable branch of the Columbia."  This eliminated the Marias from consideration as the true Missouri or route to the Pacific.
On the other hand, the Indian information obtained during the previous winter, combined with the Mackay journal and Evans map, argued strongly in favor of the southern branch:
they [the Indians] informed us that the water of the Missouri was nearly transparent at the great falls, this is the case with the water of the South fork; that the falls lay a little to the South of sunset from them; this is also probable as we are only a few minutes [of latitude] North of Fort Mandan and the South fork bears considerably South from hence to the Mountains; that the falls are below the rocky mountains and near the No[r]thern termination of one range of those Mountains, a range of mountains [the Little Belts] which apear behind the S. mountains [the Highwoods] which appear to terminate S.W. from this place and on this side of the unbroken chain of the Rocky Mountains [the Big Belts] gives us hope that this part of their information is also correct. and there is sufficient distance between this and the mountains for many and I fear for its much too many falls. another impression on my mind is that if the Indians had passed any stream as lame as the South fork on their way to the Missouri that they would not have omitted mentioning it; and the South fork from it's size and complexion of it's waters must enter the Ry. Mountains and in my opinion penetrates them to a great distance, or els whence such an immence body of water as it discharges: it cannot proceed from the dry plains to the N. W. of the Yellow Stone river on the East side of the Rocky Mountains for those numerous large dry channels which Ice witnessed on that side as we ascended the Missouri forbid such a conjecture: and that it should take it's sources to the N. W. under those mountains and travels of Mr. Fidler fo[r]bid us to belive. 
This was a brilliant piece of deduction from a fuzzy set of facts and illustrates, as well as any other event during the course of the Expedition, the competence and intelligence of the two officers.
Investigation had borne out the tentative conclusion made by Lewis and Clark on the very first day at the junction. But as before, the men remained obdurate in their belief that the northern branch was the true Missouri. The captains tried to impress their geographical concepts and reasoning on the men but Peter Cruzatte, an old Missouri hand and the party's most expert riverman, "had acquired the confidence of every individual of the party [and] declared it as his opinion that the N. fork was the true genuine Missouri and could be no other." 
Discipline had been remarkably good on the trek from the Mandan villages to the Missouri–Marias junction. Nevertheless, this was a delicate situation. Although Lewis noted that the men were ready to follow us any wher we thought proper to direct,”  the seeds of a possible breakdown must have existed. "Finding them so determined in this belief," wrote Lewis:
and wishing that if we were in an error to be able to detect it and rectify it as soon as possible it was agreed between Capt. C. and myself that one of us should set out with a small party by land up the South fork and continue our rout up it untill we found the falls or reached the snowy Mountains by which means ice should be enabled to determine this question prety accurately. 
This decision to split the party again does not indicate the captains' lack of assurance in their conclusions nearly as much as it indicates the necessity to maintain the confidence of their men.
The afternoon of the 9th was spent in making preparations for caching equipment prior to a departure from the junction and, in the evening, the party enjoyed a ration of grog distributed by the officers and danced and sang to the rhythms of Cruzatte's fiddle. June 10 was a fair, dry day and work on the cache continued.
Although Lewis was down with dysentery, it was decided that he and four men would depart the camp early the following morning, leaving Clark and the remainder of the party to complete the cache and the repair of the canoes and then to follow Lewis by water up the left-hand fork. Hopefully, before Clark and the main body could proceed too far up the south fork, Lewis would have found the proof they all desired.
At 8:00 a.m. on the morning of June 11, Lewis and his men swung their packs onto their shoulders and proceeded along the right bank of the Missouri, following Clark's earlier route. The march of the 11th was a short one and camp was made early as Lewis's illness grew more severe and he was unable to proceed. But he healed himself with a concoction made from the bark of chokecherry bushes and by morning, feeling quite revived, resumed the ascent of the Missouri.
Their route on the 12th carried them through level and open plains above the river and, after a side trip to the river for rest and refreshment during the morning, the party reached a ridge of land considerably higher than the rest of the plains. From here they saw the "august spectacle" of the Little Belt and Big Belt ranges to the south and southwest and Lewis's suspicions that they were nearing the Great Falls were confirmed. They did not reach that vital landmark on the 12th, however, and camp was made before sunset in a clump of cottonwoods along the Missouri.
After breakfasting on venison and fish, Lewis and his small band again ascended the hills beyond the river and continued their travel across the open plains. The river took a sharp bend to the south and, fearing that he would miss the Falls if he continued through the plains, Lewis altered his southwesterly course near Square and Crown Buttes and headed directly for the river.
About noon on June 13, Captain Meriwether Lewis found the proof he sought. After moving through a beautiful meadow above the river, his ears were met with the sound of falling water and his eyes with a column of spray that rose like smoke above the plains. The roaring noise increased and became too great "to be mistaken for any cause short of the great falls of the Missouri." 
Lewis hurried down the hills "to gaze on this sublimely grand specticle" and, from a position atop some rocks, he came in sight of one of the great unknowns of the West.
Here at the Great Falls of the Missouri all the information he and Clark had collected, all the assumptions they had they had made, must have seemed correct. The presence of the Falls fulfilled Lewis's dreams and ambitions and, unwilling to leave before having a look at what lay beyond the cataracts, he determined to make camp in the vicinity for the remainder of the afternoon and evening. His hunters killed buffalo during the afternoon while one of the men fished in the Missouri and that evening they feasted sumptuously on buffalo hump and fine trout.
The next morning Lewis dispatched a courier to Clark with a message dated "from the Great falls of the Missouri," and set out himself up the river to find the extent of the break in navigation created by the Falls. Cascade after cascade met his eyes and, apparently not worrying about the difficulties the long and extensive rapids would place in the way of their navigation toward the Passage to India, Lewis wrote glowingly of the views that presented themselves. He passed an eagle's nest which the Indians had told him lay near the upper end of the Falls and once beyond he climbed to the top of a hill and toward the south saw the Missouri running a meandering course toward the southwest.
From the west came the Medicine River (now the Sun River) the Indians had described to him and, on the horizon, was the second snowclad range of the Rockies. All the components of the image and the Passage to India were there and Lewis turned back to rejoin Clark and prepare for the glory that must be beyond.
The great falls, the mountains, the Medicine River—all were as the captains had expected to find them. And when, on June 16, Lewis and Clark met at the camp Clark had made at the base of the Falls in preparation for the portage around them, they must have been convinced that the easy Passage to India would also be fulfilled. Their geographical knowledge had proved accurate thus far. It should prove accurate in the future and they could expect to reach the Three Forks, the half-day portage across the final range of mountains, and then the Columbia and the Pacific—the ultimate goal of American exploration.
But disappointment and failure lay ahead. The Pacific would be reached—but not via the short portage. The Passage to India was not there. All their seeking would not reveal it and, with very few exceptions, they were the last that looked.
The dream of the Passage would end above Great Falls and the quality of the explorers' information and the accuracy of their images would begin to fade. They would learn these things in the months ahead and after their hungry and tortuous struggle through mountains that were supposed to be easy and through mountains that were not supposed to be there at all, after the constant drought and bad food of the dusty Columbian plains, after their sodden and dreary winter on the Oregon coast, they would turn their backs on the dream—on the promise of the Golden Chersonese and the breezes from the Spice Islands and the golden sands of Cathay.
But when these intrepid men turned their backs on the dream and on the Pacific, they would begin to fill in the map of the West. The Passage to India would be no more—for when man fills his maps there is little room left for his dreams.
A native of Laramie, Wyoming, John Logan Allen reflects his professional interest in the geographical history of the American West in the present article. Now assistant Professor of Historical Geography at the University of Connecticut, Storrs, he spent the 1970–71 academic year at the Clark University Graduate School of Geography, Worcester, Mass., on a post-doctoral fellowship endowed by the National Science Foundation. He offered a seminar in American attitudes toward the West during the 19th century and took time out as well to revise his doctoral dissertation for book publication. The dissertation, written for Clark University in 1969, deals with the formation of geographical images of the West front 1673 to 1804 and the impact of these images on the course and objectives of the Lewis and Clark Expedition. Before going to Clark for his doctoral work, Allen took both B.A. and M.A. degrees (with honors) from the University of Wyoming at Laramie.
This article is reprinted with the courtesy of John Logan Allen.