Having made friends with the natives and taken the observations necessary to determine the position of their villages, Thompson prepared to depart. As the Mandans dwelt in the territory of the United States, and it was contrary to the treaty of 1794 for a British company to plant trading posts among them, commerce was possible only if they were willing to make the journey to McDonnell's house to trade. Accordingly a chief in the prime of life, together with four young warriors, was selected to accompany Thompson on the trip home. These were joined by an old man and his squaw, who said that they wished to see the houses of the white men before they died. But the heights of the Missouri were too much for the aged couple, and they dropped out. Fourteen days of storm and tempest on the open plain sufficed to kill the spirit of two of the warriors, and they also dropped out. With the remainder, Thompson arrived at Assiniboine House on the 3rd of February, 1798. Unfortunately, the attempt to open up trade with the Mandans turned out to be unsuccessful. The journey was long and difficult in winter; and, in summer, the Sioux were active and cut off parties who tried to make the passage across their land so that the Mandan villages proved to be beyond the reach of the merchants from Canada. At McDonnell's house, Thompson prepared the maps and notes of his survey to be sent by the next convoy to headquarters at Grand Portage. He then took leave of his hospitable friend, and set forth on his explorations up the Red river. With three French Canadians and an Indian guide, he started down the Assiniboine, hauling his baggage and provisions by dog sleds on the ice. The tedious windings of the stream and the ever increasing depth of snow made progress difficult. Nevertheless, by the 7th of March he had reached the junction of the Assiniboine and the Red rivers, where the city of Winnipeg now stands. He then turned up the Red river, and in six days' travel came to the international boundary, beyond which it was his duty to warn all traders that they were trespassing in the territory of the United States. By the 25th of March he had worked his way south as far as the post of Baptiste Cadotte, which was situated on a tributary of the Red river, near the present site of the town of Red Lake Falls. From this point he purposed to cross the height of land to the western end of Lake Superior. Could he do it before the break-up of winter, and while the mantle of snow still lay on the ground to give passage to his dogs? He had, it will be remembered, no experience of southern latitudes. His last winter had been spent by the shore of Reindeer lake, where the ice stood firm till a windstorm broke it up on the fifth day of July. Cadotte warned him that the season was too far advanced; but he took the risk.