But the purchase of Louisiana altered matters. It was not only a matter of trade, but one of sovereignty. A double movement was initiated: one to ascend the Mississippi under Zebulon M. Pike, and the other the Missouri under Captain Meriwether Lewis and Lieutenant William Clark. The reports of these two expeditions indicate how firm a grip the English traders had upon the Indians of the upper Northwest. The expedition of Lewis and Clark ascended the Missouri and passed over the mountains to the Columbia River which was followed to the coast. The first winter, from late in October, 1804, to early in April, 1805, was spent in a fort which was constructed in the village of the Mandans, near the location of the present city of Mandan in North Dakota. Here was abundant opportunity to investigate the fur trade. Nor had they long to wait. On the 27th of November, seven British traders arrived from the North West Company's post on the Assiniboine River to barter with the river tribes. The next day, in council with the Mandan chiefs, the Americans warned the Indians not to receive medals or flags from the foreigners if they wished to be friends with the "Great American Father". A day later this warning was communicated to the traders themselves who promised to refrain from any such acts.[8] How well they kept their promises later events showed. The Lewis and Clark expedition was only a passing pageant; for by the time of the War of 1812, the only American traders who ventured to do business on the upper waters were practically driven off by the foreign companies.[9] The report of Zebulon M. Pike indicates that conditions were much worse on the upper Mississippi. Leaving St. Louis on August 9, 1805, he returned to that place on April 30, 1806. About two months were spent at a fort erected near the site of Little Falls, where he left a few men and pushed on with the rest of the company to Leech Lake. Conversation with the fur traders and councils with the Indians revealed the extent of the commerce of the North West Company. He heard of permanent trading posts on the south side of Lake Superior and at the headwaters of the St. Croix River; and he saw at Lower Red Cedar Lake, Sandy Lake, and Leech Lake the rude stockades and log buildings which were called forts.[10] These three posts were included in the "Department of Fond du Lac" and were the centers from which in the year 1805, trade with the Indians was carried on by one hundred and nine men.[11] By means of the rivers and portages of the wilderness the furs were brought to Canada without passing a custom house, and thus the United States was defrauded of duties which, it was estimated, would amount to $26,000 annually.[12]