Fortunately Meriwether Lewis was able to put together the expedition as Jefferson then visualized it, almost free from the bedevilments of urgency. As the future would show, he could not handle certain harassments. But the breaking point did not come during the Napoleonic crisis. First, military pressure in the Mississippi Valley dwindled. The initial contingent of French troops in Santo Domingo was scythed down by yellow fever and frenzied black resistance. Because of the disaster, the fleet Napoleon had designed for occupying New Orleans was shifted to Santo Domingo. The French, it seemed clear, were not likely to move into the Mississippi in the near future. That being true, the British might not be tempted to counterattack out of Canada. And so the need for immediate reconnaissance faded.
Another benison was the cooling off of the Federalists in Congress. Hoping to undercut Jefferson in his political stronghold beyond the Appalachians, the minority party scorned as too little and too late the administration's handling of the deposit affair at New Orleans. They'd end the matter once and for all, they boasted. With Senator Ross of Pennsylvania leading the charge, they introduced resolutions calling for the raising of five million dollars and fifty thousand militiamen for seizing New Orleans from Spain before the transfer to France could be completed.
Appalled, Jefferson's Republicans retorted that so thoughtless an action would precipitate a general war. Besides, the proposal was doubly illegal. It usurped the president's prerogative to handle foreign affairs, and it ignored laws that stated militia could not be sent outside the nation. After considerable hot rhetoric, the resolutions were voted down. At snail's pace Congress returned to discussing the acts Jefferson had recommended in his secret message of January 18—acts for extending both the Indian trading houses and the external commerce of the United States. The measures were passed late in February. 
Officially this left Meriwether Lewis preparing to take ten or twelve chosen men westward to test the possibility of drawing furs out of Canada into the United States. There was no tearing hurry about it—not now that military needs had relaxed—but there was a marvelous increase in kinetics. The moment Congress agreed to send a cohesive body of American troops, however small, into the ambiguously held lands beyond the Mississippi, possibilities multiplied. An American West: fantasy's new Garden of the Hesperides, where sturdy yeomen would hold true to Jefferson's vision of democracy. Scientific inquiry: the range of the wonders that might be found would set the American Philosophical Society on fire. Commerce: not only in the interior of the continent, but also with busy ports at the far end of some Western river—the fabled passage to India, dreamed of ever since the unexpected land barriers of the New World had frustrated Columbus. All this Jefferson had contemplated for years without making an overt move. Now the time had arrived. Lewis, the expeditor, must have shared the excitement of these almost boundless expectations.
Boundless, but never impulsive, never ill-considered. Even as his ideas grew, Jefferson continued speaking in terms of fewer than a dozen explorers. As late as July 2, 1803, when Henry Dearborn, secretary of war, formally authorized the project, he specifically limited the size of the party, officers included, to twelve.  It was Lewis who fudged upward in the memo of requirements he wrote out for himself—fifteen rifles, fifteen knapsacks, fifteen overcoats, and (multiples of fifteen) thirty shirts, thirty pairs of socks, and so on. 
The fifteen men would have to be, by his definition, unmarried, robust, persevering, and dependable. Some would have to be hunters, for the party would live almost entirely off the land. Carpenters, ironworkers, and gunsmiths would be welcome, if such talents were available. They might not be. The nation's shrunken army paid its private soldiers five dollars a month, plus another five dollars as a clothing allowance. Such rates did not appeal to skilled men.
Lewis wrote of his requirements to the commanding officers of Fort Massac on the Ohio, Fort Kaskaskia on the Mississippi, and South West Point, near Nashville, Tennessee. (For some unstated reason he thought South West Point would turn out to be the most fruitful source.)  To tempt good men into his special service he authorized his correspondents to offer double pay fortified by a promise that those who made the trip could leave the army as soon as the journey was over, though the normal enlistment period was five years. On being mustered out, each would also receive several hundred acres of Western land. If these inducements failed to produce qualified applicants within the army itself, the officers could recruit civilians, offering each the standard twelve-dollar bonus the army paid men who joined the ranks. Except possibly for interpreters, no one could stay a civilian and go along. This was a military undertaking, a hangover perhaps of the original nature of the project. Besides, Meriwether Lewis, professional soldier, believed that success could be guaranteed best by men working under tight army discipline. It was the beginning of a long series of military explorations launched by the United States government.
Finally, Lewis insisted on the right to interview and turn down the volunteers who had been conditionally signed on by the other officers. These choices were vital, and ultimate decisions had to be his.
Having set the size of the party, he made out, over and over again, shopping lists for equipment. Hunting shirts, kettles and cutlery, powder, axes, adzes, chisels, "muscato curtains" (mosquito netting). There had to be barrels and boxes for storage, each with its waterproof covering, and forty yards of oiled linen cut in such a way that it could be used for either tents, coverings for canoes, or sails for a keelboat.
Medicinal supplies were another essential. There was no thought of taking a doctor along. Army regulations allowed only one "surgeon" to each forty-five men. Though the president might have bent the rule, he chose not to. He had lost several of his family, including his wife, to diseases that the doctors had seemed, in his mind, to aggravate rather than cure. Nature, he wrote his friend Caspar Wistar, was the best physician. Men should not experiment "on a machine so complicated and unknown as the human body and a subject so sacred as human life."  He would make an appointment for Lewis to consult with Dr. Benjamin Rush of Philadelphia about health rules, but once the group was in the field Meriwether and his ensign, if any, would have to diagnose, prescribe, and administer on their own.
Indian presents clearly would consume more of his initial funds than any other class of supplies. The gifts were not bribes, but unconditional statements, in the Indians' minds, of goodwill and friendship.  So down went itemized notations about blue beads and brass buttons, favored as ornaments by the natives; kettles, knives, awls, rings, and burning (magnifying) glasses; tobacco, cloth, and sewing needles; ear trinkets, vermilion, and strips of copper and sheets of tin that could be converted into adornments. He also planned to take along three hand-turned mills for grinding corn, a first step in the civilizing process. Plus several peace and friendship medals of varying sizes and designs, to be passed out to deserving chiefs as tokens of esteem from President Jefferson. The more powerful the chief, the bigger the medal.
For transporting this material he planned on obtaining, at South West Point, a keelboat about sixty feet long, with an eight-ton burden. This would be backed up by a forty-foot "canoe." (He meant pirogue—canoe was a misnomer he used consistently. Pirogues were hewn from huge tree trunks; they were far heavier and harder to maneuver than the Indians' birchbark canoes or the nimble craft used by today's recreationists.)
He would float his pirogue and keelboat, loaded with soldiers and supplies, down the Cumberland River to the Ohio, continue to the Mississippi, and ascend that stream to the Missouri, long regarded as the gateway to the West. 
After his keelboat and pirogue had ascended the Missouri as far as possible, then what? Some theoretical geographers believed that only a short portage—perhaps as little as half a mile—separated the headwaters of the Missouri from those of the Columbia. If that proved true, the pirogue (though not the big keelboat) could be carried and skidded across the divide. The pirogue alone, however, might not be enough for moving the men and supplies to the Pacific. If trees were available along the portage, additional dugouts could be constructed wherever needed. If . . . but he had read somewhere that many miles of treeless plains bordered the upper Missouri. As a precaution he therefore lovingly designed an iron boat frame that could be disassembled into sections and carried in the keelboat until needed. The pieces could then be portaged, along with the pirogue, to the nearest west-flowing stream and there sheathed with either bark or buffalo hide, according to availability. If the portage proved unexpectedly long, then the iron boat alone would have to suffice for the last leg of the journey.
So far he had contemplated, in the main, physical equipment only. But he also had to acquire skills so that during the trip he could gather the kind of scientific knowledge Jefferson wanted. As an example, let us use a book to which the president and his secretary gave concentrated attention: Alexander Mackenzie's Voyages from Montreal . . . through the continent of North America to the frozen and Pacific Oceans in the years 1789 and 1793 . . . The two volumes were not published until 1801. Another edition, slightly revised, appeared in Philadelphia in 1802. Quite possibly neither version would have been written if Mackenzie had not felt the need of a powerful propaganda weapon for forcing the British government into revising the nation's fur trade. The point was not lost on Jefferson, but our consideration of the book's ax grinding needs to wait until after we look at some of Mackenzie's intellectual successes—and failures—in fields that deeply concerned the American president.
The Scottish-born trader's primary goal was commercial. (So, ostensibly, was Lewis's.) Hauling supplies by birchbark canoe from Montreal to the foot of the Canadian Rockies was costly and laborious. Great savings would result if Mackenzie could find a navigable river opening into the Pacific, one that could be used for importing trade goods and exporting fur. He made his start, in 1789, from Fort Chipewayan, built near the chill west end of Lake Athabasca primarily for this purpose. Unhappily for himself, he picked the wrong river to probe—today it bears his name, the Mackenzie—and he ended, completely dismayed, among ice floes in the Arctic Ocean. Though he carried a sextant with him, he was not adept at determining latitude with it, and he could not measure longitude at all. Consequently he returned to Fort Chipewayan with only the haziest ideas of where he had been, scientifically speaking. He had learned, however, that between Lake Athabasca and the Arctic Ocean there was no feasible outlet to the Pacific.
To remedy his navigational deficiencies he put his trade in order and then spent the winter of 1791–92 in London studying surveying. He returned to Fort Chipewayan during the spring and summer of 1792 (as Andre Michaux may well have learned while in Montreal that same season). As soon as the ice broke in 1793, he would try again, this time along a more southerly river, the Peace.
His crew was about the same size as the one Jefferson later proposed—six French voyageurs, two Indians, and an able second-in-command, Alexander Mackay. (Jefferson may well have picked up his notions about size from Mackenzie's account.) Crowded with their supplies in a twenty-five-foot birchbark canoe, the group fought their way up the howling Peace, crossed a high, narrow divide, and descended to a mighty river the local Indians called Tacoutche Tesse. It ran wildly south. Today we know it as the Fraser. But when Mackenzie learned, a little later, that an American seaman named Robert Gray had sailed his ship Columbia, on May 11, 1792, into a regal stream farther south, Mackenzie thought he had stumbled onto a higher reach of the same river, named Columbia after Gray's ship.
Warned by the Indians that he could not negotiate the crushing rapids of the Tacoutche Tesse's lower gorge in a canoe, Mackenzie cached the fragile craft for use on his return journey and led his men west, walking at first and then using an Indian canoe. When at last they reached salt water—not the open sea but a long sound—he melted some animal fat he had with him and stirred in a generous pinch of the kind of vermilion Indians used for painting their faces and the parts in their hair. With this red mixture he inscribed on a cliff face, "in large characters, 'Alexander Mackenzie, from Canada, by land, the twenty-second of July, one thousand seven hundred and ninety three,' " a statement Jefferson and Lewis must have read with envy.
More to the point, Mackenzie used his astronomical instruments and skills to find and report, in the Voyages, the approximate location of his vermilion boast. Before that, he had fixed the latitude and longitude of Montreal; of Fort William on the northwest shore of Lake Superior, where his boisterous fellow traders of the North West Company held their annual rendezvous; of Fort Chipewayan; and of the narrow divide—a mere 817 paces, or about half a mile—that separated the watershed of the Peace from that of the Tacoutche Tesse (the Columbia, as he believed). After defining those points and hundreds in between he had been able to draw, and publish in the Voyages, a map that anyone who wished to cross the continent by canoe that far north could follow.
Jefferson wanted Lewis to produce an equally reliable map of a more southerly and, the president hoped, a far less difficult crossing to the Pacific, for by that time the western sea had ceased being a target of opportunity and was emerging as the expedition's primary goal. This meant training the young man, as Mackenzie had been trained, in the arts of celestial observation. Jefferson could have served as tutor. His book Notes on Virginia had contained a map originally produced by his father and later revised by the son. He had been chairman of the committee that had prepared the first draft of the Continental Congress's famed Ordinance of 1784, which had laid down the patterns by which the public lands of the United States were to be surveyed before being opened for sale to purchasers. Of necessity he had done rough and ready surveying around his plantation. We are told, moreover, that out of sheer intellectual curiosity, he often lugged his instruments onto the roof at Monticello, took sightings on a nearby mountain, and, using that as a focal point, happily calculated the latitude and longitude of the surrounding points of interest. 
Skills, then, he had. Time was something else. His presidential duties were pressing, and his instruments—none could be had in those days in the nation's swampy capital—were at Monticello. Rather than have them wrapped and sent to him, he taught Lewis theory out of books and invented problems whose solutions depended on the young man's mastery of the printed tables and abstruse formulas used by navigators. Actual practice would have to come later, under the critical eyes of trained astronomers located in busier intellectual centers than Washington. 
Alexander Mackenzie's scientific education had gone no farther than practical astronomy. Candidly he stated in the preface to his Voyages, "I do not possess the science of the naturalist." Moreover, the toils and perils of his traverse had not given him time "to dig in the earth [or] . . . to collect the plants which nature might have scattered on the way." Lewis would do things differently. He already was gifted with acute powers of observation and a knack for describing in accurate detail what he saw. He knew enough of the flora and fauna in the eastern part of the United States to ignore similar items in the West and thus could concentrate on what was new and different. But he had a folk approach to natural history, not the scientific one that philosophers would expect when he discoursed on the new world's climate, soils, minerals, birds, animals, plants, forests, fishes, and fossils.
Again Jefferson could have served as tutor if time had allowed. His interest in the physical scene, like that of many cultured men of the time, was obsessive. The philosophy of the Age of Enlightenment pervaded his thinking. The universe, he believed, was an orderly place. A scientist's job (and there was more than a mite of elitism in this, a factor that appealed to Lewis) was to discover that order and fit it into the systematized patterns being developed by the learned men of the time—for example, by the great Swedish botanist and taxonomist, Karl Linné, who Latinized his name to Carolus Linnaeus so that it would accord with the Linnaean system of classification he had originated.
Jefferson fit the mold. He solicited, planted, and carefully noted the development of seeds and cuttings sent him by as many acquaintances in different parts of the world as he could reach. On being elected president of the American Philosophical Society in 1797, he took to his inaugural meeting, as exhibits for his address, a parcel of ancient bones that had been exhumed in western Virginia and sent him for study. He had deduced from the creature's claws that it had been a mighty feline, and he proposed naming it Megalonyx: big lion. Actually, as his friend Caspar Wistar showed, the remains were those of a huge, extinct ground sloth. No matter. Because of Jefferson's pioneering work in paleontology, the species was given his name, Megalonyx jeffersonia, much as he had been honored a few years earlier by having an American species of the European twinleaf called Jefersonia diphylla.  But occasion did not serve for presidential tutoring, and Lewis was scheduled to receive instead a series of crash courses from some of the country's leading savants in Philadelphia.
Finally, underlying and informing all else was the psychological problem of maps. For the sake of his logistics, Lewis needed to accumulate all possible data about distances, topography, and potential enemies he would face on his way west. With Jefferson offering suggestions from time to time, he pored over charts purporting to contain enlightenment. Yet at the very moment of doing this he knew that much of what was offered was based on nothing more than guesswork, dimly understood Indian tales, or academic logic concocted as a substitute for actual observation. On occasion he must have felt completely adrift: how could he stake his success on the reliability of the very charts he was supposed to correct during his travels?
He began his course in map reading with a geographic preconception that had long been a fundamental in American thinking: The gateway to the setting sun was the Missouri River. This axiom had sprung in part from the sheer power of the river as it flooded into the Mississippi—"like a conqueror," to borrow the words of the early French commentator, Pierre Francois Xavier Charlevoix, who had passed the junction in 1721. Surely such a river came from distant sources, gathering power as it moved east. 
Logic added its persuasions. The majestic Ohio and the twin tributaries that formed it, the Monongahela and the Allegheny, provided natural highways, broken by only minor carrying places, from the Mississippi Valley to the Atlantic seaboard. Did it not stand to reason that the equally majestic Missouri would provide comparable routes to the Pacific?
There were Indian tales as well. A notable one occurred in a History of Louisiana written by the Charlevoix cited above. In the spring of 1721, at the tiny French mission of Notre Dame de Cascasquios (Kaskaskia) on the Illinois bank of the Mississippi, he encountered a woman of the Missouris tribe who told him, "The Missouri rises from very high and very bare mountains, behind which there is another river, which probably rises there also and runs westward." Charlevoix's History was a popular book and went through several French editions and some in English. Jefferson considered the account necessary for good libraries to own, and it is easier to believe that Lewis read it than to suppose he did not. 
Once the far western river entered men's imaginings, it began receiving names. An early one came from the pages of a different History of Louisiana, this one published in 1763 by Antoine Simon le Page Du Pratz, who had lived beside the Mississippi for twenty-five years. During part of that time an Indian whom he called Moncacht-Apé enthralled him with tales of traveling far up the Missouri and then disembarking to wander happily north across a fertile plain, or perhaps over a low plateau, to a beautiful river flowing gently to the Pacific. (The Ohio, perhaps not coincidentally, was also called La Belle Riviêre by the French.)
Now note: Moncacht-Apé did not go west over a height of land to La Belle Riviêre, as one might assume. No, he walked north, at right angles to the Missouri, until he reached the dreamy river. In other words he (or Du Pratz) acted on the ancient concept of interlocking rivers, that is, the existence of neighboring, parallel streams that flow in opposite directions. Thus if a traveler discovered a short portage from an upbound course to a downward one, he could save many weary upstream miles. No wonder Moncacht-Apé called his "discovery" La Belle Riviêre. Jefferson read about the adventure in a copy of Du Pratz's History he purchased in Paris. Meriwether Lewis picked up an English edition in Philadelphia and carried it with him to the Pacific and back."
Du Pratz's name La Belle Riviêre for the Great River of the West (which itself is a name) was soon supplanted by Ourigan, or Ouragan, or Aurigan, or (as it eventually became) Oregon. The source was Jonathan Carver's Travels through the Interior Parts of North America in the Years 1766, 1767, and 1768, published first in 1778. Carver had fought in the French-Indian Wars under the famed ranger Robert Rogers, and afterwards the two had gone to the Great Lakes area to embark in the fur trade. Out of their work came Indian rumors (so Rogers claimed) of a great Pacific-bound river called Ouregan. Carver swiped the name and fitted it into his Travels, which sold several editions on both sides of the Atlantic. Jefferson read the tales avidly, and Lewis was surely familiar with them. 
But was there really such a river or was it the product of centuries of vaporing? Alexander Mackenzie's 1793 discovery of the Tacoutche Tesse seemed to be the first solid evidence. Hadn't he been knighted for his work? Besides, corroboration came belatedly from two more sources, an English captain, George Vancouver, and an American ship trader, Robert Gray. In 1792 while coursing the Northwest coast in search of sea-otter pelts that could be exchanged in China for tea, spices, and porcelain, Gray had noticed through the mist a set of formidable breakers crashing across a bar that masked the entrance to what was probably a large estuary. Other traders and official explorers, both Spanish and English, had noticed the same tumult but had not wanted to confront it. Gray, though, bulled across and found himself in the mouth of a huge river that he named Columbia, after his ship. He spent upwards of a profitable week trading with the river's Chinook Indians and then sailed north to the west coast of Vancouver Island. There he encountered one of Britain's great navigators, George Vancouver, for whom the island would be named, and told him about the river. Vancouver was one of those who had seen the breakers but had sailed by. Now he decided that as an official explorer of Great Britain, he had better take a more careful look.
The rough waves and shallow water of the bar beat his flagship back, but a small tender commanded by Lieutenant W. R. Broughton broke through. This was October 1792, and the water was low. Gray had been in the estuary during the high water of spring five months earlier, and his descriptions did not tally with what Broughton saw. Convincing himself that the Yankee had never left the saltwater estuary and hence could not be credited with discovering whatever river entered the huge sound (actually Gray had sailed thirty-six miles upstream), Broughton decided to take a hand in the expansion of the British Empire. He ordered some of his men to lower a longboat and row him a hundred miles east and south to the approximate vicinity of today's Portland, Oregon, well within sight of the magnificent snow cones of Mount Saint Helens and Mount Hood, which he named. In the bright moonlight of October 30, 1792, he landed and claimed possession of the entire watershed for his king and country. 
Jefferson and Lewis learned all this when the six liberally illustrated volumes of Vancouver's Voyage of Discovery to the North Pacific Ocean were published in 1801, the same year Mackenzie's book appeared. They had heard of Gray's discovery, of course, but details were maddeningly vague, for neither the trader nor his mate, John Boit, had yet published a word or drawn a map. Vancouver's work removed the uncertainties and convinced the president and his secretary that a great River of the West did exist, its mouth scientifically located by a trained navigator. Meriwether Lewis was excited enough that he copied, from Vancouver's book, charts depicting the course Broughton had covered in reaching a landmark he had named Point Vancouver. 
An unspoken but unsettling corollary followed. The River of the West, La Belle Riviêre, the Oregon, the Tacoutche Tesse, the Columbia—whatever it was called—was up for grabs. Spain definitely owned California and might insist that its province extended far enough north to embrace the stream. Mackenzie, it was generally thought, had established a British claim to the upper river and Broughton to its lower reaches. Though Gray had preceded both Britons by a small margin, the effectiveness of his short stays was disputable. A visit by a party traveling overland might help strengthen the American claim to the Northwest, even though occupation rather than mere declarations of possession were considered paramount in disputes over sovereignty. 
Another fragment of reality was added to the continent's geography by definite proof, published in 1801 and 1802, that the Missouri River extended much farther north than had been supposed. The key was the earthen, riverside villages occupied by the Mandan and Hidatsa Indians. Bold French explorers, led by the brothers Vérendrye, had visited those settlements in 1738, during a very premature attempt to reach the Pacific overland. After their withdrawal, darkness closed in until a small, loosely organized trade based on Canada's Assiniboine River was introduced during the late 1780s. By chance, Spanish traders out of St. Louis reached the villages shortly thereafter (more of that later) and conflict developed. In order to determine whether the villages lay in Spanish or English territory, the North West Company of Canada sent its chief astronomer, David Thompson, south to make the necessary observations. 
He started out in December 1797 with nine men, "fine, hearty, good humoured . . . fond of full feeding, willing to hunt for it, but more willing to enjoy it"—eight pounds of fresh deer, elk, or buffalo meat per man per day. Hoping to do some independent trading, the nine took along four or five heavily laden sleds pulled by thirty wolfish dogs. (Thompson rode a horse.) Temperatures that fell to 32°F below zero stretched a ten-day journey to thirty-three days. For Thompson there were compensations. He had his instruments with him and used the delays to determine "the latitude of six different places and the longitude of three, on the Road to the River." 
During his short stay with the Mandanes, as he spelled the name, he grew disgusted with their sexual practices (not so his men; they had agreed to make the trip partly so they could enjoy the rewards of the flesh), but he swallowed his scruples, made anthropological notes about everything he saw, and questioned his hosts in detail about the course of the Missouri above and below the villages. In the process he determined to his own satisfaction that the villages lay in Spanish territory. Whether or not his English superiors would thereafter keep their men out of the area was not his to determine. In February 1798, he returned with his party to their starting point.
Copies of the charts Thompson drew were sent by his company to London. Alexander Mackenzie saw them and used them to indicate, on the map he issued with his Voyages in 1801, the short stretch of the Missouri with which Canadian traders were in contact. Another recipient of a copy was Arrasmith, a conscientious and able British cartographer who had made a huge map of North America in 1795 and who published thoroughly revised versions in 1801 and 1802. Finally, Edward Thornton, British chargé d'affaires in Washington, apparently got hold of yet another copy of Thompson's map of the Mandan area and loaned it to Thomas Jefferson. Almost certainly Meriwether Lewis traced the chart while waiting for Congress to pass the appropriation bill that would launch the Western reconnaissance. 
Those maps, obtained almost simultaneously, gave Lewis the approximate longitude and latitude of three key points along his hypothetical route: St. Louis, the Mandan villages, and the mouth of the Columbia . . . handles on space, so to speak. Because Arrowsmith had known the direction of the Missouri's flow as it left the Indian towns and also as it drove across the present-day state of Missouri into the Mississippi, he had felt free to draw the rest of its lower course with confident, if generalized, strokes. West of the villages, however, Lewis was faced with tantalizing choices that derived, in large part, from an Indian called The Feathers.
The Feathers was a Piegan of the Blackfoot Confederacy. Somewhere in the wastelands he had run into Peter Fidler, a surveyor for the Hudson's Bay Company, who himself was supposed to have ranged south along the Rockies as far as the forty-fifth parallel of latitude, the present boundary between Wyoming and Montana. Using primitive media of some sort—a twig in the earth, a charred stick on a buffalo hide—The Feathers drew for Fidler a chart of the headwaters of the Missouri. It was a crude representation of many channels flowing east, like the veins of a leaf, into one main stem.  Fidler sent this data, along with his own observations, to the Hudson's Bay Company. The company allowed Arrowsmith to see them. He refined them to fit his conception of how a river should take shape and transferred the result to the revised 1802 map that Meriwether Lewis studied with almost agonizing care.
A little above the Mandan towns, according to Arrowsmith's conjectures (which he labeled as such by using dotted instead of solid lines), the river forked into two tributaries fed by several smaller streams, as The Feathers had suggested. The tributaries gradually spread apart like the legs of a large, reclining isosceles triangle. A ridge, already known as the Rocky or Stony Mountains, formed the base of the triangle. High points along this mountain base were named: the King in the north, the Heart midway, the Bear's Tooth in the south. The ridge as a whole was represented as rising, on the average, 3,520 feet above the plain. The altitude was comparable to that of the Blue Ridge Mountains of Virginia, and Lewis probably visualized them as such—heavily forested and creased by many steep-sided rills. It is unlikely that he imagined even the King or the Bear's Tooth as barren peaks, like those described to Charlevoix by the Indian woman at Kaskaskia. Yet did he never wonder why the Western uplift was called the Rocky Mountains?
The Missouri's northern tributary, to which Arrowsmith gave no name, headed at the peak called the King. This brought it close to one of the branches of the South Saskatchewan; this meant it might be used to further Jefferson's plan of siphoning furs out of Canada. On the western side of the King and drawn with a solid line was a fragmentary rendition of a stream called Great Lake River. A dotted line suggested that the Great Lake River continued southwest to join the southern reaches, also portrayed with dots, of Alexander Mackenzie's Tacoutche Tesse. After continuing south for some distance, the combined rivers bent east to join the Oregan (sic), whose lower course had been charted for Vancouver by Lieutenant Broughton and hence had been given a solid line by Arrowsmith.
An exciting legend appeared beside the presumed lower reaches of the Great Lake River: "The Indians say they sleep 8 Nights in descending the River to the Sea." Only eight nights after crossing a pass north of the King—a pass that probably did not exceed the half-mile portage Mackenzie had found up where the land was reputedly sterner. Here, indeed, was a way to the Pacific!
But was it any better than the southern branch?
Arrowsmith believed the southern fork to be the main one and had labeled it "River Mississury." If it carried more water than the unnamed north fork, it might be more easily navigated. There was another point. Though the River Mississury ran due west from the Mandan villages to the Bear's Tooth, it received several large southern branches that flowed out of a section bearing the note, "Hereabouts the Mountains divide into several low ridges." So it might be possible for Lewis's party to work through those low hills to some southern tributary of the Oregan. This would enable the explorers to stay in American territory, whereas following the north fork might cause them to trespass on British soil. And if reaching the Oregan by the southern branch proved impossible, they might try the Colorado River instead—it was shown as rising no great distance to the south—and float down it to the Gulf of California and the Pacific, an option Jefferson himself had suggested. 
Secretary of the Treasury Albert Gallatin added to the uncertainties. If Lewis carried with him all the available maps of the trans-Mississippi region, the packet would be unwieldy. Why not have Nicholas King, official surveyor of the city of Washington, combine all relevant data into a single master chart? 
King agreed, but in transferring Arrowsmith's data to his own map, he made several revisions. Some arose from different interpretations of the David Thompson material, and some, it would appear, from King's (and Lewis's?) own flights of fancy. He thought the north tributary, left unnamed by Arrowsmith, was the main one, for he labeled it "Missesourie" (it, too, was portrayed with dotted lines) and he shifted the Great Lake River far enough south so that a portage between the two streams would not encroach on British territory. He renamed the dotted southern tributary, which Arrowsmith called the main stem, as the "Lesser Missesourie." He wiped out the low hills south of the Bear's Tooth. Most intriguing of all, he showed a conjectural great southern fork of the Oregan (even he, an American, disdained Gray's name Columbia) sweeping around the southern foothills of the Bear's Tooth, to rise well east of the Lesser Missesourie's headwaters. That is, the Lesser Missesourie and the south fork of the Oregan (if it existed) now interlocked. Lewis's explorers, following the example of Du Pratz's Indian, Moncacht-Apé, could walk readily from one stream to the other without crossing a major divide. Moncacht-Apé, of course, had walked north from the Missouri to La Belle Riviêre, but it is not beyond the realm of possibility that the nature of the feat was in King's mind when he produced a similar situation south of the Bear's Tooth. And it was probably thoughts of the Oregan's southern tributary and of possible approaches to the Colorado River that led Jefferson to urge, in his instructions, that Lewis keep a sharp eye on the Missouri's southern confluences. 
Choice. Meriwether Lewis had at his fingertips two speculative maps prepared by two of the world's best cartographers from the latest information available. He could not ignore them. Yet which route, if any, could he depend on to take him and his party safely and successfully to their destination? Had not the great Alexander Mackenzie made an almost fatal choice that had spun him off to the sterile ice of the Arctic in 1789? Lewis could afford nothing like that.
These plans for transcontinental exploration were more daring in 1803 than they seem now. France still owned Louisiana and might continue to do so; America's envoys, Livingston and Monroe, had been instructed to dicker only for New Orleans and West Florida. Yet Jefferson wanted to push an American commercial highway across that foreign territory into another area—the Oregon country—to which title was by no means clear. More complications arose from Alexander Mackenzie's recently published Voyages. The Scot was less interested in obtaining book royalties than in bending the British government to his will. He concluded his tale by urging Parliament to charter a huge monopoly that could control the entire fur trade of Canada. The proposed firm would introduce goods into the heart of the continent by way of Hudson Bay in the East and the Columbia, over which Britain must spread its sovereignty, in the West. The new company would have the right to dispose of its Western fur harvest in the profitable markets of the Orient without interference from the South Sea or East Indian monopolies. Faced with so much power, America's exploitive ship captains, currently the dominant force in the sea-otter trade of the Northwest Coast, "would instantly disappear," or so Mackenzie argued. Even the beaver trade of the upper Missouri would be lost, for Mackenzie also proposed moving Canada's boundary with Louisiana Territory south from the forty-ninth parallel to the forty-fifth. If all this came to pass what point would there be in the American president and his secretary nosing along speculative river courses and over dreamed-of portages, searching for the Pacific?
There is no way to explain the president's audacity except to believe he was deliberately playing the odds. If the French did occupy Louisiana, there was a possibility they would send the same kind of expedition to the Pacific he was contemplating—and he certainly wanted to beat them to that, for the sake of American glory if nothing else.  But there was a much better chance that Napoleon would be unable to move into Louisiana. His armies were stalled in Santo Domingo, and he faced, in Europe, a renewal of his desperate wars against England, as Jefferson correctly sensed. (The new conflict broke out in May 1803.) So the president was willing to move ahead on the assumption France would not—could not—interfere.
What of Spain? No formal transfer of territory had taken place. Hoping perhaps that none would because of Napoleon's other involvements, the Spanish government might prove as intransigent as Yrujo had predicted when Jefferson had approached him earlier. In that event, Madrid's longstanding policy of using Louisiana as a buffer to protect Mexico might lead the officials in New Spain to disregard France's ownership and resist Lewis's "invasion" with force. But New Spain was, in the main, lethargic, and Jefferson was willing to take that chance, too.
Then there were the Britons, whom Mackenzie was trying to stir into more empire building in the Northwest. The British were not averse to such thinking, but in order to turn Mackenzie's blueprint into actuality, Parliament would have to tread on the toes of three powerful monopolies, the Hudson's Bay, South Seas, and East India companies. That the government would do this for a private entrepreneur's hustling company seemed unlikely to Jefferson. Furthermore, Mackenzie's own partners in the North West Company were reluctant, as the president may have known, to support the Scot's expensive dream, even though some of their traders were already groping for a new way across the Rockies from the headwaters of the Saskatchewan.  So Jefferson did not look for immediate objection there, either, and, in fact, probably warmed himself with the thought that by getting Americans to the Columbia ahead of the British, he could throw yet another roadblock in Mackenzie's way.
He concealed his audacity by lying about his purposes. Immediately after Congress had passed and he had signed, on February 28, 1803, the act appropriating money for extending the commerce of the United States, he asked the British and French ministers to grant passports to Lewis's party.  He assured both diplomats that only the language of the act was devious. Lewis was furthering science, not commerce or territorial acquisition, and would carry with him only such merchandise as he needed to mollify suspicious Indians. The act spoke of commerce, the president said, only because the American Constitution forbade the government's financing enterprises devoted to advancing science or promoting geographical discoveries.
Such statements were a calculated deception. Though Jefferson did earnestly wish to promote science and geographical knowledge, he also hoped to draw as many furs out of the British area of influence as possible. Developing good will among the Indians while learning trade routes and navigable river systems was essential to the realization of that mercantile goal.
Only the French minister, Louis André Pichon, raised an eyebrow. Inasmuch as Spain still administered Louisiana, he said, was its minister, Carlos Martinez, Marqués de Casa Yrujo, also granting passage? Jefferson evaded the question, saying only, "Qu'il devait le donner": he ought to grant it. He went on with a great show of candor, tracing out Lewis's proposed route on Arrowsmith's map and then lending Pichon the British passport so he could use it as a model in preparing his own.
A final and very strange bit of camouflage was his telling, and ordering Lewis to tell, anyone who noted preparations for the expedition and wondered about its goal that it was bound up the Mississippi.  The French and British ministers knew better, as did the Federalists in Congress who had seen the enabling act passed. So why this last cover? No one can be sure. Possibly the president hoped to keep Federalist newspaper editors from raising a commotion about the true nature of the exploration. Perhaps he worried lest Western frontiersmen, feeling they could go wherever an official government party went, might create an incident by trying to follow Lewis into Louisiana. Or perhaps he believed, with his usual predilection for secrecy, that any questionable enterprise needed concealment as a matter of course and acted accordingly.
Whatever the reason, seldom has an expedition of such momentous potentials been prepared for with so little fanfare.