Lewis selected as his first assistant, and to act as leader in case of his own disability or death, William Clark, four years his senior, and a brother of George Rogers Clark, who had captured Kaskaskia, Cahokia, and the Illinois country, and otherwise distinguished himself. Strangely enough, in his mental and moral qualities William Clark was almost a duplicate of Meriwether Lewis. Throughout the whole of the hazardous, difficult, toilsome journey that was now begun, the two men were most devoted friends, Lewis having not the slightest fear that Clark would receive too much credit, and neither having the least jealousy of the other; quite in contrast with some expeditions of later years, where the leader must be all in all. The result was that the world to-day exalts all the more this noble commander-in-chief because his broad generosity forever linked with his, almost as a single name, that of his subordinate officer, so that the great undertaking is not the Lewis, but the Lewis and Clark, expedition. Clark was to have a commission as captain, but when it came it proved to be merely lieutenant. He took it, never grumbled, and, when he returned, gave the commission back.

As far as the head of the Missouri apparently they would traverse no absolutely new ground, for as has been noted the Frenchmen had been from the Mandan villages to the eastern slopes of the Rocky Mountains, and from the French-Spanish settlement of St. Louis numbers of trappers and traders had gone to the Mandan country, while the chief road to that region had long been from the north-east by way of the lakes, and the Assiniboine where the British fur companies had established trading-posts. From these points their traders reached the natives of the upper Missouri and Mississippi, which territory the British claimed as their own. Charles Chaboillez, one of the chief factors of the North-west Company, was in charge of Montagne à la Basse, situated on the Assiniboine probably about where the Verendryes' Fort la Reine had been. From here he and other traders often went to the Mandan country to deal with various natives who made that region a rendezvous. He had proposed to Daniel Harmon that they should make a journey from the Mandan towns west to the Rocky Mountains,[65] over the course the Mandans "pursued every spring to meet and trade with another tribe ... which resides on the other side of the Rocky Mountain." This expedition was never undertaken, but it indicates the degree of familiarity possessed by the Mandans with the Western country, and shows how the earlier Frenchmen found their way out at least as far as the Yellowstone, already known as the Roche Jaune, a name which in itself is a suggestion of early French visitors to the great falls of that stream where the gorgeous yellow colouring is so remarkable. Peter Fidler, another of the British fur traders, had been down from the Saskatchewan through the area drained by the headwaters of the Missouri. A trader named Cruzatte had a post in 1802 at a point two miles above old Council Bluffs, so that all through this eastern portion of the Wilderness white men had scatteringly penetrated. The French had been on the Saskatchewan before the eighteenth century was half over, and ten years before the American purchase of Louisiana Mackenzie had crossed to the Pacific by way of Peace River Pass. But beyond the Rocky Mountains no one appears to have been away from the coast besides Escalante coming up from Santa Fé to Utah Lake, and Mackenzie from Fort Chepewyan to the shore of the Pacific at King Island (lat. 52° N.), so that the region Lewis and Clark designed to enter beyond the mountains was absolutely unknown territory, outside of the Amerinds themselves.

Mount Hood—From Cloud Cap Inn.

From Wonderland, 1903—Northern Pacific Railway.

Captain Lewis was ready to start on this traverse before the official transfer of the Louisiana region from France to the United States had been made. His plan was to go to La Charette, the farthest French settlement up the Missouri, a few miles above St. Louis, and there spend the winter of 1803-04, the season being so far advanced that it was not considered advisable to make the final start till spring opened again and they would have a chance to go as far as possible before another winter began. But the Spanish officers there objected to Lewis's entering the territory and a camp was made on the American side of the river, about opposite the mouth of the Missouri. The purpose of the expedition was communicated to the foreign ministers, and passports obtained from France and Great Britain. Spain was particularly jealous of this movement or any other which led to crossing the Mississippi, and had opposed the right of the United States by the Louisiana Purchase to anything more than the region around New Orleans and the mouth of the Mississippi; they resented Napoleon's selling even that. Hence Spain looked upon the upper country as still hers. The situation, considering the indefinite character of the whole transaction, was full of disagreeable possibilities. Perhaps this was Napoleon's intention. Therefore Jefferson in his instructions to Captain Lewis particularly says: "If a superior force, authorised or unauthorised, by a nation, should be arrayed against your further passage, and inflexibly determined to arrest it, you must decline its further pursuit and return." The British fur companies, jealous of each other, were still more jealous of encroachments on their trading grounds and their attitude toward the expedition was uncertain.

On the 30th of April, 1803, the treaty with France ceding Louisiana was consummated, and on June 20th Lewis's instructions were signed and he departed for La Charette by way of Pittsburg and the Ohio.[66] Congress ratified the purchase on October 17th the same year. Travelling was mainly by water in those days, and that seems to be the reason why Lewis stuck to boat travelling when horses would have been so much easier across the western prairies. They were permitted to return by sea, if necessary and possible, and Lewis carried letters of credit which would have obtained money for him in any port, or on any ship, the world round. The object of the exploration as announced was to find a waterway across the continent, but Jefferson doubtless had more in view than such a diplomatic statement would imply. Intercourse with natives, he particularly directed, should be friendly and conciliatory.

With forty-two men and three boats Lewis and Clark left their winter quarters on May 14, 1804, and proceeded up the Missouri, passing the village of St. Charles, with a French Canadian population of 450, and a little above it a small group of American farmers. On the 25th they passed La Charette, the last settlement, and were then fairly under way. Two rafts were met June 12th, on one of which was a trapper named Dorion who had been for more than twenty years among the Sioux. They engaged him to go with the party and this increased the number to forty-five all told. There were nine young men from Kentucky, fourteen American soldiers who had volunteered, a French interpreter, a French hunter, and a negro belonging to Clark. All but the negro were enlisted as privates and drew pay from the Government. Besides these there were a corporal and six soldiers, who were to turn back at the Mandan towns, and nine voyageurs. One boat was fifty-five feet long, drawing three feet of water, with a square sail and twenty-two oars, and was armed with a swivel at the bow. There was a deck of ten feet at the bow and stern, while the middle was covered with lockers which could be raised to form a breastwork. The other boats were open, one having six and the other seven oars. Two horses were taken along the bank. Had they abandoned the boat idea and taken to horses they might have gone to the mouth of the Columbia while they were going to the Mandan towns, but they did not know how great a bend the Missouri makes. They pushed steadily up the river, meeting many natives and having friendly relations with them, and the last of October arrived at the Mandan towns, having consumed five and a half months in making the ascent.

As soon as a proper place was found they felled trees and built houses for the winter, calling the place Fort Mandan. The Mandans were perfectly familiar with white men, as has been noted, and were mainly peaceable. Lewis found here one of the British traders, McCracken, and by him sent a note to the chief of his home post, Chaboillez, also enclosing a copy of his British passport. This apparently was to avoid friction with the fur companies, and it indicates the uncertain condition of the claim of the United States to the territory he intended to traverse. Lewis evidently concluded a month later that he was quite within American territory, for he forbade Laroche,[67] another British trader, from presenting medals and flags to the natives. Up to this time the expedition had lost but one man. Sergeant Floyd, who died near where Sioux City now stands, and he was the only man who was lost on the whole expedition. Nor were any seriously injured but Captain Lewis, who was accidentally shot in the thigh by one of his men who had poor eyesight and took him for an elk. This occurred on the return not far above the mouth of the Musselshell, and Lewis was well before they reached St. Louis. The freedom from needless accident is strong proof of the judgment which Lewis and Clark used in the management of the party, for disasters are usually the result of wrong decisions.